perf(scanner): fast-path identifier characters and short-circuit keyword scans

[DylanPiercey]

Ports the expression fast paths from marko-js/htmljs-parser#223 (commit 3c95d7f) to the external scanner:

• Identifier/number-character fast path in the expression loop — a word character is never whitespace, never a terminator, and never a switch case, so it skips the per-character termination checks (including should_terminate's eager lookahead) and the switch dispatch and just advances.
• Keyword-scan short-circuit — look_behind_for_operator / look_ahead_for_operator bail out immediately when the boundary character can't start/end a keyword (every keyword is lowercase ASCII).

No behavior change — the 372-fixture comparison suite passes identically. Measured ~6–9% faster on expression-heavy input in an A/B build (matching the ~6% reported upstream).

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Generated by Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 35bcb231-b1bd-40f8-9b4f-c46a3d4de153

📥 Commits

Reviewing files that changed from the base of the PR and between dd3f38d and 9e15dd6.

📒 Files selected for processing (1)

• src/scanner.c

Walkthrough

src/scanner.c now short-circuits unary and binary operator keyword matching when the inspected character is outside lowercase ASCII. The expression scanner also adds a direct path for characters that satisfy is_word_code, advancing past them before the termination and switch-dispatch logic.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main scanner performance change: fast-path character handling and keyword scan short-circuiting.
Description check	✅ Passed	The description is directly related to the changeset and matches the reported scanner fast paths and performance goal.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/htmljs-parser-tree-sitter-fixes-dm8wj0

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