fix(gc): explicit gc() forces the conservative native-stack scan (#4977)

[proggeramlug]

Fixes #4977.

Problem

Explicit gc() (a full collection with the default auto stack-scan mode) skipped the conservative native-stack scan: gc/roots.rs::conservative_stack_scan_decision_for maps Auto → SkipDisabled. At a gc() callsite, live module-init/top-level locals can be held only on the native stack — neither the precise shadow-stack roots nor the module-var scanners cover them — so the collector reclaimed the whole object graph and later field reads returned dangling-pointer garbage. Silent corruption, no crash:

const keep = { nested: { deep: "leaf-string-4916" } };
gc();
console.log(keep.nested.deep); // garbage (reused string memory, e.g. "globalThis")

Fix

• gc/roots.rs: new ManualGcScanGuard — pins the Full conservative scan for the duration of a manual collection, only when no per-thread override is already pinned. The GC unit tests pin Auto inside their controlled-root scopes (so forced collections still reclaim objects held only as native-stack test locals) and keep working unchanged; an explicit PERRY_CONSERVATIVE_STACK_SCAN env value beats any override either way, so the bisection escape hatch is intact.
• gc/policy.rs: both manual-collect paths — direct js_gc_collect and the deferred-flush Collect(Manual) arm — now share one manual_gc_collect_now() helper that engages the guard (they previously duplicated the weakref + collect sequence).
• gc/heap_snapshot.rs: the Diagnostics fakes: v8 heap snapshot is an empty-but-valid graph; inspector/repl sessions look real but aren't #4916 workaround that wrapped its collect in the Full override is superseded and removed; js_gc_collect now provides the guarantee.

Threshold-triggered automatic collections are intentionally untouched — the Auto skip exists for copied-minor eligibility and per-cycle cost; this change scopes the full scan to explicit collections where the caller observably holds live state at the callsite.

Validation

• Repro (test-files/test_issue_4977_gc_toplevel_locals.ts, object literal + class instance): prints 16 / leaf-string-4916 / widget-name-4977 / 1 with the fix; previous binary printed 10 / globalThis / <empty> / 1.
• PERRY_CONSERVATIVE_STACK_SCAN=0 still reproduces the skip (env precedence verified at runtime); PERRY_GEN_GC=0 legacy path unaffected.
• RUST_TEST_THREADS=1 cargo test --release -p perry-runtime gc:: → 377 passed / 0 failed, including the new manual_gc_scan_guard_forces_full_scan_only_when_unpinned covering both the unpinned-engage and pinned-respect cases.

Code-only PR — no version bump / changelog (maintainer folds metadata at merge).