Implement private async-context call ABI

Parent: #134

Implements the coherent async-context call boundary shared by the pinned Home and Bun inventories. This is a child of the private ABI coverage work tracked by #140, #163, and #164.

Scope

• AsyncContextFrame__withAsyncContextIfNeeded
• Bun__JSValue__isAsyncContextFrame
• Bun__JSValue__call

Bake__getAsyncLocalStorage is intentionally out of scope: upstream exposes a separately stored Bake/dev-server ALS instance, not the currently active async-context value captured by these wrappers.

Acceptance

• Add a realm-local active async-context value as a precise traced and relocated GC root.
• Represent an AsyncContextFrame as a branded, non-callable object with strong callback and captured-context edges.
• Return callbacks unchanged when no context is active and avoid double-wrapping existing frames.
• Invoke plain callbacks and wrapped callbacks with exact argument ordering and global-this fallback for an empty receiver.
• Install the captured context for the dynamic extent of the call and restore the prior context before normal return or exception publication.
• Reject invalid/foreign handles without crossing VM ownership boundaries.
• Cover brand, capture, nested restoration, throw restoration, GC retention/relocation, and compiled Home/Bun consumer signatures.
• Refresh generated ABI inventories and coverage documentation, batch Debug/ReleaseSafe verification, then close this issue only after the pushed main commit is green.

Implemented and pushed directly to main in df79596 (feat(abi): implement async context calls (#373)).

Delivered:

• AsyncContextFrame__withAsyncContextIfNeeded, Bun__JSValue__isAsyncContextFrame, and Bun__JSValue__call
• realm-local precisely traced/relocated active async-context root
• branded null-prototype non-callable frame with strong callback/context edges
• exact inactive identity, no double wrapping, encoded argument order, empty-receiver global fallback, same-VM validation, and restoration before normal return or throw publication
• insertion barriers plus a temporary root for the displaced context across callback-triggered GC
• compiled Home and Bun consumer evidence and refreshed pinned inventories/docs

Verification (Debug and ReleaseSafe):

• focused async-context engine tests: 3/3 passed, 0 leaked
• relocation-focused engine tests: 25/25 passed, 0 leaked
• Home private consumer: 352/352 linked and runtime matrix passed
• Bun private C-API extension consumer passed
• C/API inventory audit passed

Coverage is now Home 365/471 (106 pending) and Bun 357/461 (104 pending). Bake__getAsyncLocalStorage remains intentionally separate because upstream stores a distinct Bake/dev-server ALS instance.