fix(kotlin): extract extension functions with a generic or nullable receiver (#3608)

[squid-protocol]

Closes #3608.

Problem

Kotlin func_start and args only accepted a plain [\w.]+ receiver. These were extracted as nothing:

• fun Collection<CodeBlock>.joinToCode(...)
• fun <T> List<T>.b()
• fun Map<String, Int>.c() = 1
• fun String?.d()

Fix

func_start and args now share one _RECEIVER fragment. It accepts:

• a generic argument list, using the one-level-nesting idiom (Map<K, List<V>>), with bounded repetition so an unclosed < stays linear;
• an optional nullable ?.

Verification

• kotlinpoet, every .kt file, against tree-sitter function_declaration: recall went from 96.4% to 99.0% (2034 → 2090 of 2111). That is +56 functions and 0 new false positives: every newly extracted name is a tree-sitter function in the same file.
• Tests: new valid, invalid and ReDoS cases in tests/extraction/languages/test_kotlin.py. The kotlin tests pass (150).
• Golden masters: crucible_check.py --update --yes reports no drift in either fixture; the crucible's Kotlin samples have no generic receivers.
• Other gates:
  • tree_sitter_accuracy_audit --lang kotlin --ci: OK.
  • import_graph_accuracy --ci: OK. Kotlin stays at 100/95.2. The remaining misses are extension properties (val KmClass.isObject), which are not functions and are out of scope here.
  • ruff: no new findings. mypy: kotlin.py has none. The only mypy findings locally were yaml stubs and re.sub overload issues in untouched files, caused by this environment.

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[squid-protocol]

tree-sitter-accuracy-audit failed on javascript (files_scanned: baseline=18 current=19). This PR didn't cause it: main has been red on the same check since #3627.

Bisect:

• 5d32971f: passes.
• c2d1f1b5 (fix(auditor): an import manifest is not a packed payload (#3583) #3627): fails, and the Kotlin commit isn't involved.

Cause: #3627 stopped the Packed Payload Guard from relegating import manifests. The crucible's three.js barrel threejs/Nodes.js (166 lines, 139 of them export … from) is scanned again. It declares no functions or classes, so every other count is unchanged.

Fix: the JavaScript baseline is regenerated in 6ab22bb on this PR. main's check goes green again once this PR merges.

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