Extraction hardening: go #817

Part of the extraction-hardening epic (#813). See
tests/extraction/how_to_harden_extraction.md
for the full methodology this issue follows.

Language: go

Known finding to fix first

func_start doesn't recognize Go 1.18+ generics at all (e.g. func Foo[T constraints.Ordered](a, b T) T {) -- mainstream, ~5-year-old Go syntax currently invisible to the engine. The regex captures the name then goes straight to [ \\t\\n]*\\( with no allowance for a [...] type-parameter list in between.

Rules in scope (non-None in LANGUAGE_DEFINITIONS["go"]["rules"]):

• func_start
• args
• class_start
• _dependency_capture

Checklist (per the methodology doc, for each in-scope rule above)

• valid: modern idiom + legacy/historical syntax eras + testing-framework-shaped functions
• invalid: string-literal lookalikes, type-level lookalikes, assignment-in-condition, macro-expansion lookalikes, and any language-specific lookalike shape
• pathological: 10-15 cases covering annotation/attribute stacking, nested generics/templates, destructured/complex params, multi-line splits at every plausible boundary
• Every case empirically verified against the real compiled regex before being added
• Any genuine bug found is fixed (full ReDoS/lint/type/crucible discipline) or filed separately if it's a multi-language architectural fix
• Migrated out of the four old monolithic dict files into tests/extraction/languages/test_go.py
• Epic updated with any new recurring-bug-class findings before closing this issue