feat: gate GC_init() and -lgc behind usesGC

[cs01]

Summary

• Auto-detect GC usage via emit() override in LLVMGenerator — any instruction containing @GC_ sets usesGC = 1 without manual tagging at 250+ call sites
• Pre-scan fallback in generateParts() scans irParts and globalStrings before generateMain() to catch GC references from string-builder helpers (ir += patterns) that bypass emit()
• Gate GC_init() behind usesGC in generateMain() — only called when program uses heap allocation
• Gate GC declarations behind gc: flag in getLLVMDeclarations() — declare void @GC_init(), declare noalias i8* @GC_malloc(i64) etc. only emitted when used
• Gate -lgc behind usesGC in both compiler.ts and native-compiler-lib.ts
• Explicit tagging in json.ts::generateJsonParser() for json parser functions that build IR via lines.push() rather than ctx.emit()

Status

The infrastructure is complete and correct. For programs that use path/fs/string helpers (which are unconditionally emitted and reference @GC_malloc), usesGC is still always true — these helpers need to be gated in a follow-up PR before "hello world" can skip GC entirely. The linker/IR gate correctly responds to usesGC and will automatically skip GC once those helpers are gated.

Test plan

• All 426 tests pass (node + native)
• Hello world compiles and runs correctly
• JSON parse test compiles and runs correctly
• Self-hosting stage 0 failure is pre-existing on main (unrelated to this PR)