Extract anchor functionality into standalone mdbook-anchorez preprocessor

[Copilot]

mdbook-judgment handled both judgment rendering and {anchor} code-snippet embedding — the anchor piece is useful enough to publish independently. This splits it out into its own crate, mdbook-anchorez.

New mdbook-anchorez crate

• Scans source files for // ANCHOR: name / // ANCHOR_END: name markers
• Replaces {anchor}`name` references in markdown with rendered <figure> HTML blocks
• Configurable via [preprocessor.anchorez] in book.toml:
  • src-dir — directory to scan (default: "src")
  • github-base — base URL for [src] links (default: "", omits the link)

[preprocessor.anchorez]
command = "cargo run -p mdbook-anchorez --"
github-base = "https://github.com/dada-lang/dada-model/blob/main"

mdbook-judgment cleanup

• Removed Anchor struct, parse_anchors, dedent, render_anchor, anchor scanning, and the {anchor} replacement pass
• SourceIndex now only holds judgments

Workspace / config

• Added mdbook-anchorez to workspace members in root Cargo.toml
• Added [preprocessor.anchorez] to book.toml

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