feat: support custom repo/branch for acestep-cpp via env vars

[Copilot]

build.sh was hardcoded to clone from audiohacking/acestep.cpp with no way to target a fork or specific branch without editing the script.

Changes

• build.sh

  • Added ACESTEP_CPP_BRANCH env var (empty → default branch) and --branch NAME CLI flag
  • git clone conditionally passes --branch "$BRANCH" when set
  • Updated usage comment to cross-reference both flags and their env var equivalents

• setup.sh

  • Prints effective repo URL and branch before invoking build.sh
  • Build failure hint now includes ACESTEP_CPP_REPO / ACESTEP_CPP_BRANCH usage examples

• .env.example

  • New acestep-cpp source / custom fork section documenting both vars

Usage

# Custom fork + specific branch
ACESTEP_CPP_REPO=https://github.com/your-fork/acestep.cpp.git \
ACESTEP_CPP_BRANCH=experimental \
./setup.sh

# Or passed directly to build.sh
./build.sh --repo https://github.com/your-fork/acestep.cpp.git --branch experimental

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