fix(ts): contextually type imported overloaded object-literal calls

Parent: #548

A strict two-file reduction adds an ordinary callable overload before the mapped-handler overload fixed by #666:

export type Handler = (item: Item, rewritten: boolean) => Item;
export type Handlers = { [K in Kind]?: (item: Item<K>, rewritten: boolean) => Item };
export declare function visit(handler: Handler): Item;
export declare function visit(handlers: Handlers): Item;

For visit({ a: (item, rewritten) => ... }), TypeScript 6.0.3 and the native TypeScript preview select the object-handler candidate for contextual typing, infer Item<"a"> and boolean, and report only deliberate TS2322/TS2339 controls. ReleaseFast Home emits TS7006 for both callback parameters and misses both controls. Removing the first overload makes the same program pass under #666, isolating overload-candidate contextual typing rather than mapped-key substitution itself.

The same pattern appears in the pinned Zod 4.5.2 graph: visit has callable and mapped-handler overloads, while classic and mini in-out.ts retain 16 TS7006 diagnostics across mapped visitor callbacks.

Acceptance criteria:

• contextually type object-literal arguments against applicable imported overload candidates
• retain #666 concrete mapped-key specialization inside the chosen contextual candidate
• preserve overload order, ambiguity behavior, unrelated generic parameters, and optional mapped members
• retain negative TS2322 and TS2339 controls; do not suppress errors or widen callbacks to any
• add a permanent imported overload positive/negative regression
• match TypeScript 6.0.3 and the native preview exactly on the standalone oracle
• run full checker and Program suites, ReleaseFast, and an exact unchanged Zod parent/candidate identity audit with zero additions
• document any admitted corpus change in the performance record and README

Implementation landed on main in b6f5eef4a (fix(ts): contextually type imported overload objects (#667)).

What changed:

• Program schemas now transfer multiply declared functions as an ordered intersection of their bodyless overload signatures. A concrete implementation body is deliberately excluded from the public call surface.
• Contextual object-literal checking filters arity-compatible overloads by declared property ownership only. It selects a signature only when exactly one candidate remains; callback values are not checked during selection, so ordinary overload resolution remains authoritative.
• Mapped property domains use the existing constraint relation and preserve the concrete mapped-key specialization added by #666.
• No package, symbol, property, or benchmark-specific special case was added.

Permanent regressions:

• Imported generic/optional first overload plus imported mapped-handler second overload: no TS7006/TS2345; exact deliberate TS2322 + TS2339 controls.
• Ambiguous same-property overloads retain first-declaration contextual order, matching TypeScript.
• A property owned by no overload remains uncontextualized (TS7006), rather than selecting an inapplicable signature.

Validation:

• Program suite: 193/193 passed.
• Full checker suite: passed.
• zig build -Doptimize=ReleaseFast: passed.
• zig fmt --check and git diff --check: passed.
• Full required Pickier run completed at the unchanged repository baseline: 22,427 existing problems (11,834 errors, 10,593 warnings); changed files are Zig-only.

Exact two-file oracle:

• TypeScript 6.0.3, native TypeScript preview, and ReleaseFast Home all report exactly the deliberate TS2322 (Item<"a"> to number) and TS2339 (missing property on boolean).
• Home reports no false TS7006 and no false TS2345.

Unchanged Zod 4.5.2 benchmark (tsconfig.benchmark.json), two candidate runs:

• Deterministic byte-for-byte candidate output: SHA-256 3212f2598cc37ba0eb8a98a14c0ddfe82158ee4797ee75ec8f5d8565d5414774.
• Baseline and candidate: 400 diagnostics, 395 unique path/line/column/code identities.
• Identity SHA-256 is unchanged: 86df9bf4b128d164b88a96d4dfa2441094dbeeaa514b2587fa73c5ac0f3cdab5.
• Exact identity delta: 0 removals, 0 additions.
• The 16 in-out.ts TS7006 diagnostics remain because Zod's real handler template includes a deeper conditional/Extract schema outside this overload-transfer slice; timings were excluded because the shared host was contended.

README and detailed benchmark documentation are the remaining acceptance item and will land separately.

Documentation is now on main in 9c13f230f (docs(ts): publish imported overload object audit (#667)). The README links the detailed audit and carries the Zod summary table.

The final same-working-directory production rerun strengthens the earlier evidence: the #666 baseline and both independent #667 candidate stderr files are byte-identical, SHA-256 0590e3b2b593d2e2c0abb32f9f732d45f6d1701f3c2f906d07975ddf39c0bbc9. The normalized identity delta remains 0 removals / 0 additions.

All #667 acceptance items are complete. The deeper conditional/Extract owner-schema gap that keeps Zod's 16 target TS7006 diagnostics is separate follow-up work; #667 did not approximate or special-case it.