ABI: own standalone VM lifecycle and GC deferral

Parent: #140 Profiles: #163, #164 Roadmap: #134

Implement the shared pinned VM ownership boundary as real zig-js semantics:

• JSC__VM__create
• JSC__VM__deinit
• JSC__VM__deferGC
• JSC__VM__setControlFlowProfiler

JSC__VM__notifyNeedShellTimeoutCheck remains outside this slice because it is jsc-shell-only timeout machinery with no zig-js shell state; its explicit rejection remains documented.

Acceptance:

• Create a standalone, refcounted context-group VM for both pinned heap-type tags and fail safely on allocation failure.
• Define exactly-once VM owner release without invalidating retained realms, and reject foreign-global misuse.
• Defer every applicable zig-js collection entry point across nested callbacks, then preserve queued async collection intent after the outermost scope exits.
• Store real per-VM control-flow-profiler state instead of a compatibility no-op.
• Add lifecycle, realm-retention, nesting, callback, collection, misuse, and allocation-failure tests.
• Add compiled Home/Bun consumer evidence in Debug and ReleaseSafe.
• Refresh both exact inventories, #140/#163/#164, and concise ABI documentation.

Completion evidence:

• VM implementation and exhaustive unit coverage: fd4cc8cd
• Consumer fixtures, CI, and documentation: fabafb81
• Focused engine slice: 6/6, zero failures/leaks
• Home consumer: 388/388 symbols in Debug and ReleaseSafe
• Bun VM lifecycle consumer: Debug and ReleaseSafe
• Current inventories: Home 408/471 (63 pending), Bun 400/461 (61 pending), zero unclassified
• Release matrix derives 124 remaining private imports.

Completed in a84af344 and a9b4d5b5. The four shared VM imports now have owned lifecycle, exact nested GC deferral, real profiler state, exhaustive allocation rollback, and Debug/ReleaseSafe Home+Bun consumer evidence. Inventories are Home 408/471 and Bun 400/461 with zero unclassified.

Post-rebase commit IDs: implementation fd4cc8cd, consumer/CI/docs fabafb81.