One garbage collector per process: TypeScriptRuntime.dll on gc.dll (Windows)

[ASDAlexander77]

Summary

Under -mm=gc, every binary that links Boehm statically runs its own collector. When two of them share a process, a collection frees objects that only the other binary's memory still references. Nothing crashes; the program reads wrong data.

TypeScriptRuntime.dll and the default library's TypeScriptDefaultLib.dll both linked Boehm statically. So a JIT run that loads the default library, or any gc DLL, had two collectors. An exe that loads a user DLL also had two, because the default-library DLL brought its own. This PR is step 1, 5 and part of 6 of tslang/docs/single-gc-collector-design.md, included here as a proposal.

• Runtime on gc.dll (Windows only).
  • The top-level CMake defines an imported tslang_gc_shared target from 3rdParty/gcdll/x64/<build>. TSLANG_GC_SHARED_PREFIX overrides the path, and GC_DLL makes the Boehm headers declare its functions as imported.
  • Configure now fails on Windows if the shared Boehm is missing.
  • TypeScriptRuntime links the new target and copies gc.dll into bin/. gc.cpp no longer forces static linking when GC_DLL is set.
• Builds and packaging.
  • prepare_3rdParty.bat builds the shared Boehm.
  • The release zip ships bin/gc.dll in its root.
  • The release workflow passes GC_SHARED_LIB_PATH to the default-library build.
• Tests. New import_gc_single_collector.ts / export_gc_single_collector.ts, registered as -jit -shared and AOT -shared. The library side builds the strings the importer holds, then churns with different content. That makes the library's own collector run, and it must see the importer's array. The existing owned-returns tests churn in the importer, which is why their JIT variant passed with two collectors.
• Docs. docs/memory-models.md and the design doc.

Companion PR: TypeScriptCompilerDefaultLib, branch fix-single-gc-collector. It links the gc TypeScriptDefaultLib.dll against gc.dll.

Linux is unchanged; nothing has been measured there yet.

Measured (Windows release)

Every "before" failure also passed with collection suppressed (GC_INITIAL_HEAP_SIZE=1GB), so the extra collector is the cause.

Case	Before	After
test-jit-shared-export-import-gc-single-collector	assertion failed	pass
test-compile-shared-export-import-gc-single-collector	pass	pass
JIT + default-library DLL, 2000 held strings	2000 bad	0 bad
exe + user DLL + default-library DLL	1984 bad	0 bad

The new JIT test was also rerun with the old static-Boehm runtime to confirm it still fails there.

Test plan

• Full ctest, Windows release: 2,704 of 2,704
• TypeScriptRuntime.dll and TypeScriptDefaultLib.dll both import gc.dll (llvm-readobj --coff-imports)
• Plain JIT and async JIT smoke tests (thread registration through gc.dll)
• CI green on Windows and Linux
• Release workflow (only runs on a tag): zip contains gc.dll in the root and gcdll/

Not in this PR

• Steps 3, 4 and 6 of the design:
  • tslang choosing the shared GC for --emit=dll and for exes that import a DLL
  • an import-time error for a DLL that carries its own collector
  • packaging docs
• A debug build of the default library against the debug shared Boehm.
• Linux.
• A separate compiler bug found here, not fixed: in a module compiled as an import, a const declared in a for body in a namespace function gives "can't resolve name". The new test uses let.

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