ABI: implement private C-API call and proxy extensions

Parent: #134 Related: #140, #163, #164

Implement the two shared private extensions declared by pinned Home/Bun javascript_core_c_api.zig.

Acceptance:

• Export JSObjectCallAsFunctionReturnValueHoldingAPILock with exact C signature, recursive VM API-lock coverage, default-global this, ordered argument conversion, and normal encoded return values.
• Convert a thrown call into an encoded exception cell returned to the caller without leaking it into the VM pending-exception slot.
• Return empty for null, non-callable, foreign-VM, or otherwise invalid values without publishing partial results.
• Export JSObjectGetProxyTarget as a pure trap-free direct target projection; return null for non-proxies, revoked proxies, non-value private handles, and null inputs. As in JSC, dereferencing a handle after its entire context group is destroyed is outside the C API lifetime contract.
• Preserve same-VM identity/lifetime and avoid invoking proxy traps.
• Add focused engine and compiled Home/Bun consumer evidence in Debug and ReleaseSafe.
• Refresh inventories/docs and parent issue evidence.

Completed in 930f177b and pushed directly to main.

Evidence:

• exact Home/Bun C signatures exported for the API-lock call wrapper and proxy target projection;
• recursive lock behavior is observed both when nested under an existing lock and from a native callback at lock depth 1;
• default-global this, ordered arguments, encoded normal returns, exception-cell returns, and an empty VM pending-exception slot are covered;
• null/non-callable and foreign function/receiver/argument values fail empty;
• proxy target identity is strict-equal, trap count remains zero, and non-proxy/revoked/non-value/null inputs return null;
• focused engine test: 3/3 pass, zero skips/failures/leaks in Debug and ReleaseSafe;
• pinned Home consumer: 348/348 symbols linked and runtime matrix passed in Debug and ReleaseSafe;
• dedicated pinned Bun consumer passes in Debug and ReleaseSafe;
• inventory audits: Home 361/471 implemented (110 pending), Bun 353/461 (108 pending), zero unclassified;
• ABI parser unit tests: 4/4 pass;
• CI now runs the dedicated Bun fixture, and ABI/API/Home integration docs carry the new counts and semantics.

The root README benchmark block is unchanged because this ABI slice does not affect or regenerate benchmark results.