Add interactive semantic diff HTML reports

[forhappy]

Summary

• add the hard-cutover compass diff OLD NEW --format html --output PATH workflow
• render a self-contained reviewer report with actionable semantic findings, exact source changes, and meaningful code-graph changes
• embed pinned @pierre/diffs 1.2.12 for unified/split layouts, line numbers, intraline emphasis, hunk metadata, and wrapping
• retain the exact Git patch as a no-JavaScript/error fallback and lazy-mount file diffs as accordions open
• normalize graph deltas, collapse non-semantic metadata and edge-identity churn, and preserve exhaustive node/edge lists in the report
• document the CLI, output contract, offline behavior, vendored license, and reproducible bundle build

Why

PR reviewers need one concise artifact that explains what behavior changed, what may break, which callers/modules are affected, what tests are missing, and how both the source and code graph changed. The previous text/JSON-only flow did not provide an interactive review surface, and a basic line-oriented patch view could not provide side-by-side or intraline review without an external tool.

User impact

Users can now run:

compass diff OLD NEW --format html --output semantic-diff.html

The resulting file is fully offline and self-contained: it has no CDN or runtime server dependency. The existing text and JSON workflows remain available; --output is intentionally valid only for HTML.

Validation

• cargo test -p compass-semantic-diff -p compass-history
• cargo test -p compass-cli --test history_cli diff_emits_semantic_text_json_html_and_rejects_removed_flags
• cargo test -p compass-cli semantic_diff_render::tests --lib
• cargo check -p compass-cli
• cargo fmt --all -- --check
• git diff --check
• generated and exercised a real bbolt report in the browser: unified/split switching, wrapped lines, lazy second-file rendering, intraline spans, exact-patch fallback behavior, and zero fresh console warnings/errors