ABI: implement stable borrowed ZigString output views

[chrisbbreuer]

Parent

• Roadmap: implement every README-declared missing runtime surface #134
• ABI: define and implement versioned Bun and Home private-JSC profiles #140
• ABI: implement the pinned Home private-JSC profile #163
• ABI: implement the pinned Bun private-JSC profile #164

Scope

Implement the two pinned borrowed-output bridges:

• JSC__JSString__toZigString for exact JSString cells.
• JSC__JSValue__toZigString with full ordered ToString coercion and VM exception publication.

Return stable VM-lifetime views using untagged Latin-1 bytes for 8-bit strings and bit-63-tagged UTF-16 code units otherwise. The cache must be shared by sibling realms, isolated by VM, and released at context-group teardown.

Acceptance

• Empty, ASCII, Latin-1, BMP, astral, and lone-surrogate strings expose exact length/tag/code units.
• Repeated views remain stable across later allocations and sibling-realm calls.
• JSValue conversion covers every primitive, object coercion order, thrown values, Symbols, foreign VMs, and first-exception preservation.
• Invalid JSString cells zero the output without exposing unrelated memory.
• Focused runtime, both inventories, README/docs, and parent trackers are current.

Completion evidence

Implemented on main in 6300bbc3; README and ABI documentation published in c13cc7f7.

• Exact empty, ASCII, Latin-1, BMP, astral, and lone-surrogate lengths, tags, and code units are covered.
• Borrowed buffers are VM-group-owned, stable across later allocations, shared by sibling realms, isolated across VMs, and released at group teardown.
• JSValue output performs observable ToString for primitives and objects, preserving order and publishing Symbols, thrown values, and foreign inputs through the VM exception slot.
• Invalid JSString cells and every failure path zero the output; a pre-existing exception remains first.
• zig build test-home-private-abi -Doptimize=Debug passes 95/95 symbols.
• Home audit: 97/431 implemented, 334 pending. Bun core audit: 90/421 implemented, 331 pending.

[chrisbbreuer]

Completed on main in 6300bbc with documentation in c13cc7f. Focused runtime: 95/95; Home: 97/431; Bun core: 90/421.