Make source view render metrics width-aware

[jmqd]

Source view content caching omits render width while comment rows and wrapped metrics depend on width. Resize or scrollbar width changes can mis-anchor comment context, and large-file scrolling recomputes full-file wrapped metrics per frame. Cache width-dependent source metrics or split width-independent lines from width-dependent row metadata.

[jmqd]

Context: source render audit found width-dependent comment rows and wrapped metrics coupled to content cache usage. Risk is both correctness after resize/scrollbar width changes and O(file-lines) work on repeated source renders. No local fix in this turn.

[jmqd]

Patch: 6286a9c includes code_width in FileSource and BlockSource content-frame cache keys so width-derived source comment rows are not reused after resize or scrollbar width changes. Regression coverage: 1eb0b13 adds source-width cache-key tests for file and block source modes. Verification: cargo test content_cache_key --lib passed 9 tests; just check passed with 982 tests, clippy, and fmt.

[jmqd]

Patch: 87c1bfa stores wrapped source display metrics in BuiltContent at build/cache time, so source renders reuse cached total row count and focus row range instead of recomputing wrapped metrics across source lines each frame. Verification: cargo test build_file_lines_source_mode_stores_wrapped_display_metrics --lib; cargo test content_cache_key --lib; just check passed with 986 tests, clippy, and fmt.