fix(dart): long-param constructors not found + assert() false-positive func_start

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Fixes #1493.

Summary

• _slice_by_braces's dart-specific terminator-scan window was pinned to start_idx + 2000, but the scan itself starts at params_end_idx -- for constructors whose parameter list alone exceeds ~1800 chars (e.g. flutter/theme_data.dart's const ThemeData.raw({...}), ~140 lines / hundreds of required this.xxx, entries), params_end_idx landed past the window, leaving no room to find the real terminator and silently rejecting the whole match.
• Fix: dart_search_limit = min(next_match_start, max(search_limit, params_end_idx + 200)) -- anchors extra room to params_end_idx only when needed, so every normal function keeps byte-identical behavior and only the pathological long-param case widens. (A flat params_end_idx + 2000 was tried first and empirically found to reach spurious terminators in unrelated code across other files -- see commit history in this branch.)
• Found during verification, fixed in the same PR (same root cause, not a separate scope): assert was missing from func_start's reserved-keyword exclusion lookahead, so assert(...); could be misdetected as a function definition named assert. This was already a widespread, pre-existing false positive in the flutter corpus (hundreds of instances across framework.dart/navigator.dart/object.dart/semantics.dart) -- not introduced by this PR, just newly reachable once the search window widened enough for assert's own terminator scan to succeed instead of timing out.

Numbers (7-file flutter/dart corpus, live audit)

• found_functions: 1096 -> 1100
• extra_functions: 571 -> 426 (25% precision improvement, mostly from the assert fix)
• ThemeData.raw and the unnamed factory ThemeData(...) constructor: now found.

Test plan

• pytest tests/extraction/languages/test_dart.py tests/extraction/languages/test_dart_strict.py -- 186 passed
• tree_sitter_accuracy_audit.py --all --ci -- clean across all 31 languages (shared-helper ripple check)
• crucible_check.py --mode both -- PASS/PASS after blessing expected drift (theme_data.dart's function count changed materially)
• audit_check.py --ci -- all clear
• New regression tests: a synthetic >2000-char-param constructor, and assert(...) non-matches

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