Skip clearAnnotation AST walk in MapGenerator.generate for parsed roots with no comments

[alanzabihi]

Hypothesis

Even with thesis #42 merged (bypass MapGenerator when no source map), MapGenerator.generate() is still called when source maps are present or when the fast-path conditions dont apply. In that case, clearAnnotation() (map-generator.js:65-88) walks the entire root.nodes array backwards looking for sourceMappingURL comments.

For Workload B, the generated CSS files have no source map annotations. The clearAnnotation walk is always fruitless but still iterates all children of root.

Add a fast-path check: if root.nodes has no Comment children (or all comments dont start with # sourceMappingURL=), skip the walk. Since the benchmark generates files without source map annotations, this check would be O(1) for the common case.

Better yet, track during parsing whether any source map comment was found. Set a flag on Root: root._hasSourceMapComment. clearAnnotation checks this flag before walking.

Editable surface

• lib/map-generator.js — skip clearAnnotation when no source map comments present
• lib/parser.js — set _hasSourceMapComment flag on Root during parsing (optional)

What's different from prior work

• Bypass MapGenerator construction for no-source-map stringify in LazyResult #42 (accepted) bypasses MapGenerator entirely when no map option. This thesis optimizes the case where MapGenerator DOES run but clearAnnotation is unnecessary.
• Eliminate per-file MapGenerator and clearAnnotation AST walk overhead in Workload B #46 tried lazy Map allocation and NoWorkResult (neutral because Bypass MapGenerator construction for no-source-map stringify in LazyResult #42 already covers the main path). This targets the remaining path.
• Replace newline regex in MapGenerator.generateString with indexOf loop #43 targeted generateString newline regex (neutral because source-map codepath isnt exercised). This targets clearAnnotation which runs on ALL generate() calls.

Expected impact

METRIC_B improvement of 1-3ms. clearAnnotation walk eliminated for 250 files.

[alanzabihi]

Polyresearch approval: thesis #72.