Phase 1: GC Interface Abstraction

[usiegj00]

Part of #1

Extract Crystal's GC calls behind an abstract interface so different GC backends can be swapped at compile time.

Scope

• Define CrystalGC module interface: alloc, alloc_atomic, collect, stats, enumerate_objects, register_stack, unregister_stack
• Implement CrystalGC::Boehm wrapping current LibGC calls
• Replace direct LibGC calls in src/gc/boehm.cr with interface calls
• Zero behavior change — all existing tests must pass

Key Files

• src/gc/boehm.cr — current GC integration
• New: src/gc/interface.cr — abstract interface

Upstreamable

Yes — this is a non-breaking refactor. Can be proposed to crystal-lang/crystal as-is.

[usiegj00]

Phase 1 implemented on branch `phase1-gc-interface` (commit 1aaf677).

What landed

• New `src/gc/interface.cr` — documents the `CrystalGC` backend contract: `alloc`, `alloc_atomic`, `realloc`, `free`, `collect`, `enable`/`disable`/`disabled?`, `stats`, `prof_stats`, `is_heap_ptr?`, `register_stack`/`unregister_stack`, `enumerate_objects`.
• New `CrystalGC::Boehm` — thin, policy-free wrappers over `LibGC`. `enumerate_objects` raises `NotImplementedError` (Boehm is conservative — honest contract; precise enumeration arrives with Immix). `register_stack`/`unregister_stack` are documented no-ops (Boehm discovers stacks itself).
• The `GC` facade now delegates all allocation/collection/introspection through `CrystalGC::Boehm`. Tracing + locking stay in the facade; thread/world integration stays Boehm-internal and will be generalized in Phase 3.
• New `spec/std/gc/boehm_spec.cr` (9 examples) pins the backend contract.

Verification (zero behavior change)

• `gc_spec.cr` + new `boehm_spec.cr` + `weak_ref`/`reference`/`fiber` specs: all pass.
• Full `std_spec.cr` type-checks (`--no-codegen`, exit 0).
• `-Dgc_none` and `-Dpreview_mt` configurations both build and run.
• `crystal tool format --check` clean.

Design note: the headline interface is routed; deeply platform-coupled Boehm plumbing (pthread_*, stop/start world, stackbottom, collection callbacks) is intentionally left in place as backend-internal rather than forced into a cross-backend data-plane interface. Documented in `interface.cr`.