Initialize project setup, enhance srcset parsing, and update docs

.github/workflows/ci.yml

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+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - develop
+      - main
+
+permissions:
+  contents: read
+
+jobs:
+  verify:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 10.30.3
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Verify
+        run: pnpm run verify
+

.github/workflows/release.yml

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+name: Release
+
+on:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: write
+  pull-requests: write
+  id-token: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Release Please
+        id: release
+        uses: googleapis/release-please-action@v4
+        with:
+          release-type: node
+
+      - name: Checkout
+        if: ${{ steps.release.outputs.release_created }}
+        uses: actions/checkout@v4
+
+      - name: Setup pnpm
+        if: ${{ steps.release.outputs.release_created }}
+        uses: pnpm/action-setup@v4
+        with:
+          version: 10.30.3
+
+      - name: Setup Node.js
+        if: ${{ steps.release.outputs.release_created }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: 24
+          cache: pnpm
+          registry-url: https://registry.npmjs.org/
+
+      - name: Update npm
+        if: ${{ steps.release.outputs.release_created }}
+        run: npm install --global npm@latest
+
+      - name: Install dependencies
+        if: ${{ steps.release.outputs.release_created }}
+        run: pnpm install --frozen-lockfile
+
+      - name: Verify
+        if: ${{ steps.release.outputs.release_created }}
+        run: pnpm run verify
+
+      - name: Publish to npm
+        if: ${{ steps.release.outputs.release_created }}
+        run: npm publish --provenance --access public
+        env:
+          HUSKY: 0

.gitignore

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+.DS_Store
+.idea/
+.claude/
+
+node_modules/
+dist/
+coverage/
+.rslib/
+.rstest/
+.rsbuild/
+
+*.log
+npm-debug.log*
+pnpm-debug.log*
+yarn-debug.log*
+yarn-error.log*

.husky/commit-msg

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+pnpm exec commitlint --edit "$1"
+

.husky/pre-commit

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+pnpm exec lint-staged
+

.husky/pre-push

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+pnpm run test && pnpm run build
+

.npmrc

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+engine-strict=false
+fund=false
+

.nvmrc

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+22
+

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+This project uses Conventional Commits and Release Please to generate releases.
+
+## 0.0.1 - 2026-05-02
+
+### Added
+
+- Initial npm package scaffold.
+- TypeScript source entrypoint.
+- Rslib dual ESM/CommonJS build with TypeScript declarations.
+- Rstest, Biome, publint, commitlint, and Husky development workflow.
+- GitHub Actions workflows for CI, Release Please, and npm Trusted Publishing.

CONTRIBUTING.md

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+# Contributing
+
+Thanks for taking the time to improve `srcset-kit`.
+
+This package is intentionally small: it parses, validates, and serializes HTML
+`srcset` values without runtime dependencies. Contributions are most helpful
+when they keep that focus clear.
+
+## Getting Started
+
+Requirements:
+
+- Node.js 22 or newer;
+- pnpm 10 or newer.
+
+Install dependencies:
+
+```sh
+pnpm install
+```
+
+Run the full local check:
+
+```sh
+pnpm run verify
+```
+
+`verify` is the same kind of check expected before a release. It runs formatting
+checks, linting, type checking, tests, the production build, and package export
+validation.
+
+## Useful Commands
+
+Use the full check before opening a pull request:
+
+```sh
+pnpm run verify
+```
+
+Use focused commands while developing:
+
+```sh
+pnpm run test
+pnpm run test:watch
+pnpm run typecheck
+pnpm run lint
+pnpm run format
+pnpm run format:check
+pnpm run build
+pnpm run pack:check
+```
+
+## Project Shape
+
+The main files are:
+
+- `src/parse.ts` for the public parser entrypoint;
+- `src/parser.ts` for internal tokenization;
+- `src/validator.ts` for validation rules and issue codes;
+- `src/stringify.ts` for serialization;
+- `src/types.ts` for public types;
+- `src/errors.ts` for package-specific errors;
+- `tests/index.test.ts` for behavior coverage;
+- `README.md` for public examples and user-facing API docs.
+
+## Public API Changes
+
+Keep the public surface small and intentional. The runtime API is:
+
+- `parse`;
+- `validate`;
+- `stringify`.
+
+When changing public behavior, update these together:
+
+- exported types in `src/types.ts`;
+- exports in `src/index.ts`;
+- user-facing examples in `README.md`;
+- tests in `tests/index.test.ts`.
+
+Please avoid adding dependencies unless the benefit is clearly worth the extra
+package weight.
+
+## Testing Guidelines
+
+Add focused tests for the behavior you change.
+
+Parser changes should cover cases like:
+
+- commas inside URLs;
+- `data:` URLs;
+- relative and absolute URLs;
+- query strings and fragments;
+- whitespace and newlines;
+- tolerant parsing of invalid descriptor sets.
+
+Validator changes should assert stable issue codes, not only messages.
+
+Stringifier changes should cover normalized output and round trips with
+`parse()` when possible.
+
+If object validation changes, add candidate-array tests in addition to string
+input tests.
+
+## Documentation Guidelines
+
+Keep README examples short, copyable, and aligned with the actual API.
+
+If a feature affects how users call `parse`, `validate`, or `stringify`, update
+the README in the same pull request. Use the repository's TypeScript formatting
+style in examples, including compact object and import braces:
+
+```ts
+import {parse, validate} from "srcset-kit";
+
+validate([{url: "image.png", density: 1}]);
+```
+
+## Commits
+
+This repository uses Conventional Commits. Examples:
+
+```text
+feat: add srcset parser
+fix: keep descriptor whitespace valid
+docs: improve validation examples
+chore(release): merge main back into develop
+```
+
+Husky runs `commitlint` for commit messages.
+
+## Branch Flow
+
+The repository follows a GitFlow-like process without requiring the GitFlow CLI:
+
+- feature and fix pull requests target `develop`;
+- release pull requests merge `develop` into `main`;
+- Release Please creates a release pull request in `main`;
+- after the release pull request is merged, GitHub Release and npm publishing run automatically;
+- merge `main` back into `develop` after every release so `develop` receives the version, changelog, and lockfile updates.
+
+When a merge commit is created manually, keep the merge message conventional:
+
+```text
+chore(release): merge main back into develop
+feat: merge feature/parser
+```
+
+## Hooks
+
+Husky hooks are installed by `pnpm install`.
+
+- `pre-commit` formats staged files with Biome through lint-staged and keeps the formatted result in the current commit.
+- `commit-msg` validates Conventional Commits.
+- `pre-push` runs tests and the build.
+
+## Release Readiness
+
+Before a release, make sure:
+
+- `pnpm run verify` passes;
+- README examples match the exported API;
+- public type changes are covered by tests;
+- `CHANGELOG.md` is ready for the release flow;
+- package metadata in `package.json` still matches the published package;
+- the license file remains `LICENSE.md`.

LICENSE.md

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+# MIT License
+
+Copyright (c): Anjey Tsibylskij
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.