ABI: implement copied, atomic, and rope string construction

[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 exact shared private string-construction boundary against pinned Home and Bun revisions:

• ZigString__toValueGC: copy every tagged Latin-1, UTF-8, and UTF-16 representation into a selected-VM JavaScript string.
• ZigString__to16BitValue: match the pinned raw UTF-8-to-WTF string conversion contract.
• ZigString__toAtomicValue: canonicalize equal strings to one VM-owned atomic string cell without leaking identity across VMs.
• JSC__JSValue__createRopeString: perform ordered ToString conversion and concatenate exact UTF-16 code-unit sequences while publishing abrupt completion through the shared VM exception slot.

External-string callback/finalization exports and borrowed toZigString views are separate ownership families and are out of scope.

Acceptance

• Every tagged input encoding covers empty, Latin-1, astral, and lone-surrogate cases.
• Copying is proven by mutating source storage after construction.
• Atomic strings converge within one VM and remain isolated across VMs.
• Rope conversion covers primitive/object coercion order, same-VM siblings, foreign values, and first-exception preservation.
• Focused runtime, both inventories, README/docs, and parent trackers are current.

Completion evidence

Implemented on main in 2bd1a7c6; README and ABI documentation published in 77bb23fa.

• ZigString__toValueGC copies empty, Latin-1, UTF-8, UTF-16, astral, and lone-surrogate input; mutation-after-copy is covered.
• ZigString__to16BitValue implements raw UTF-8 conversion and rejects invalid input through the VM exception slot.
• ZigString__toAtomicValue uses one VM-owned intern table shared by sibling realms and isolated from foreign VMs.
• JSC__JSValue__createRopeString performs left-then-right ToString, exact surrogate-pair concatenation, same-VM sibling acceptance, foreign-VM rejection, abrupt completion, and first-exception preservation.
• zig build test-home-private-abi -Doptimize=Debug passes 93/93 symbols.
• Home audit: 95/431 implemented, 336 pending. Bun core audit: 88/421 implemented, 333 pending.

[chrisbbreuer]

Completed on main in 2bd1a7c with documentation in 77bb23f. Focused runtime: 93/93; Home: 95/431; Bun core: 88/421.