ABI: implement ZigString and BunString error factories

Parent

• #134
• #140
• #163
• #164

Scope

Implement five shared pinned private error factories:

• JSC__JSValue__createTypeError and JSC__JSValue__createRangeError from ZigString message/code pairs.
• JSC__createError, JSC__createTypeError, and JSC__createRangeError from every BunString representation.

Match selected-realm intrinsic prototypes, exact message/code text, freshness, first-exception behavior, and the pinned descriptor distinction: RangeError code is read-only while TypeError code is writable.

Acceptance

• Empty, Latin-1, UTF-8, UTF-16, astral, and lone-surrogate message/code inputs are covered.
• TypeError and RangeError code descriptors match pinned attributes, including omission for an empty code.
• BunString Empty, WTFStringImpl, ZigString, StaticZigString, and invalid/dead behavior is explicit.
• Selected-realm freshness, same-VM siblings, and first-exception preservation are covered.
• Focused runtime, both inventories, README/docs, and parent trackers are current.

Completion evidence

Implemented on main in 9a34a555; README and ABI documentation published in 8bc87bcf.

• ZigString message/code pairs cover empty, Latin-1, UTF-8, UTF-16, astral, and lone-surrogate text.
• TypeError code is writable/enumerable/configurable; RangeError code is read-only/enumerable/configurable; empty codes are omitted.
• BunString Empty, WTFStringImpl, ZigString, StaticZigString, UTF-8, and dead input dispositions are covered.
• Fresh selected-realm instances, sibling realms, dead-input failure, and first-exception preservation are exercised.
• zig build test-home-private-abi -Doptimize=Debug passes 100/100 symbols.
• Home audit: 102/431 implemented, 329 pending. Bun core audit: 95/421 implemented, 326 pending.

Completed on main in 9a34a55 with documentation in 8bc87bc. Focused runtime: 100/100; Home: 102/431; Bun core: 95/421.