Preserve extracted native Object.assign identity

[CaelmBleidd]

Bundled JavaScript such as Vest stores Object.assign in a module variable (var assign_default = Object.assign) and calls it later. The method-property capture previously recognized only an immediately matching receiver call; extraction could become a mixed-kind value and stop at the correct function-field runtime-sort guard.

Give the exact native SDK Object.assign property a stable callable identity even without an immediate call. Retain its native owner/name for later unbound calls while each invocation keeps its own argument/result types and passes through the ordinary native mutation/nullish-target boundary. Captured native functions retain function typeof and can be recovered from boxed storage. The runtime-sort guard is unchanged.

Reuse the existing written/deleted property metadata so a replacement or deletion takes precedence on a later property read. Already extracted original functions retain their native target after either mutation. Project/source objects named Object keep their own method behavior. This PR does not implement Object.assign property copying; that remains the selected model/residual-call policy.

Depends on #589 for nullish native Object targets, which in turn depends on #559. This PR contains one additional callable-identity commit.

Validation: 18 focused tests pass with zero failures or skips (5 new source/native-oracle tests, 8 nullable Object-boundary tests, 5 builtin-owner tests). Strict TS main/test lint reports zero issues. Removing only the new native capture entry point makes the source alias/function-identity check return 0 instead of 1; restoring it passes. This discriminates lost native identity without claiming that this focused fixture reproduces the original corpus sort-guard exception. A separate method-bearing object-literal shadow fixture stopped before the subject operation; its exact source/hash and failed trace are preserved as a separate finding, with root cause unverified. The shadow regression here uses the existing supported declared-source-class pattern and retains the same native oracle result. Another preserved fixture exposes deferred module initialization when a module original is first read only after replacing the native property; the retention tests explicitly read the original into a local before mutation to isolate callable identity. Eager module side-effect semantics are outside this change. Full suites are left to CI; real corpus impact is checked on the combined runtime.