ABI: exact JSGlobalObject owning-VM query

Parent: #140 Tracked by: #134, #163, #164

Implement the pinned JSC__JSGlobalObject__bunVM export: the exact owning-VM query every consumer realm funnels through (bunVM(), bunVMUnsafe(), tryBunVM(), bunVMConcurrently()). The declaration is byte-identical in the pinned Home JSGlobalObject.zig (revision 7ed99c02) and the Bun core inventory; the pinned Bun C++ bridge returns the VirtualMachine registered in the VM's WebCore client data.

Acceptance

• Pin the exact declaration extern fn JSC__JSGlobalObject__bunVM(*JSGlobalObject) *VM.
• Return the one VM identity that owns the selected realm — the same pointer zig-js already publishes through JSC__JSGlobalObject__vm and consumes in every JSC__VM__* export, since zig-js's context group is both the JSC VM and the embedding VM.
• Sibling realms in one context group resolve to the identical VM pointer; a foreign context group resolves to a different VM; an invalid handle returns null rather than a stale or fabricated pointer.
• Cover identity, sibling-sharing, foreign-group distinction, and null-handle behavior in the generated compile-link-runtime fixture.
• Mark the export implemented in both inventories with zero unclassified entries, and update the README/private-ABI counts and evidence.

Design constraint

This is a pure ownership query: it must not allocate, attach, or create any VM state, and it must not disturb a pending VM exception.

Evidence

Completed on main in runtime commit 731e218f with README/private-ABI evidence in 95bc0ceb.

• The export returns the realm's context-group VM pointer, exactly the identity JSC__JSGlobalObject__vm publishes and every JSC__VM__* boundary consumes; zig-js's context group is both the JSC VM and the embedding VM, so the two Bun-side pointers coincide by architecture.
• Fixture coverage: non-null identity with JSC__JSGlobalObject__vm on the primary realm, identical pointer from a sibling realm in the same group, distinct identity for a foreign context group, foreign identity matching its own vm, and null for a null handle.
• Generated Home consumer: 281/281 symbols linked; runtime matrix passed.
• Home inventory: 279/431 implemented, 152 pending, zero unclassified.
• Bun core inventory: 271/421 implemented, 150 pending, zero unclassified.
• Public/private export audit: 117 public functions complete, 287 classified private profile exports.
• Focused c_api unit shard: 148 passed, 0 skipped, 0 failed, 0 leaked; public C and Home public ABI fixtures green (50/50).
• The committed 1,540-sample benchmark matrix remains current because this allocation-free query is outside its workloads.