ABI: implement exact Symbol registry and description bridges

Parent

• #134
• #140
• #163
• #164

Scope

Implement the three shared pinned Symbol bridges:

• JSC__JSValue__symbolFor
• JSC__JSValue__symbolKeyFor
• JSC__JSValue__getSymbolDescription

Acceptance criteria

• symbolFor decodes every tagged ZigString representation, uses the VM-shared GlobalSymbolRegistry without observable userland calls, and preserves registry identity across sibling realms while isolating foreign VMs.
• getSymbolDescription returns the exact UID/description as a stable borrowed ZigString view, writes the canonical non-null empty view for an empty description, and leaves output untouched for non-Symbol inputs.
• symbolKeyFor returns true and writes the exact registry key only for registry-backed Symbols; local and well-known Symbols return false and leave output untouched.
• Latin-1, UTF-8, UTF-16, astral/lone-surrogate, empty, local, registered, and well-known cases are covered alongside mutation-safe input copying, sibling/foreign VM behavior, invalid input, allocation failure, and first pending-exception preservation.
• Home/Bun inventories, README/API evidence, and all parent trackers are regenerated from checked-in results.

Implemented and published directly on main.

Implementation: 5f8bf8f3 (feat(abi): implement Symbol registry bridges (#205)) Documentation: e9d8de57 (docs(abi): publish Symbol bridge evidence (#205))

Delivered:

• exact symbolFor, symbolKeyFor, and symbol-description bridges;
• every tagged ZigString representation, including Latin-1, UTF-8, UTF-16, astral text, lone surrogates, and empty keys;
• stable group-lifetime Latin-1/UTF-16 output views with untouched outputs on false/non-Symbol paths;
• exact registered vs local vs well-known behavior, caller-input mutation safety, sibling identity, foreign-VM isolation, and first pending-exception preservation;
• a fix at C-API realm bootstrap so all sibling realms share one GlobalSymbolRegistry even when registration begins after both realms exist.

Validation:

• zig build test-home-private-abi -Doptimize=Debug — green, 118/118 symbols linked and runtime matrix passed.
• zig build home-private-abi-audit bun-private-abi-audit — green.
• Home: 119/431 implemented, 312 pending.
• Bun core: 113/421 implemented, 308 pending.
• Public/extension accounting remains 117/19.

README benchmark scores were preserved because this was an ABI/correctness slice and no benchmark input changed.