feat(cli): emit faithful TypeScript module declarations

[oxura]

Description

Fixes #1144.

-o core.mjs now emits core.d.mts from checked telescopes and constructor fields. Runtime and declaration export filtering share js_exports; the module retains its default object, while declaration named exports are type-only. Native Nat follows current main's number ABI (#1506), not the older BigInt example in the issue.

The declarations preserve recursive tagged values, generic datatype fields, arrays, callback shapes and every prefix of a partially applied exported function. Private identifiers are encoded separately from public type names, including reserved names. Erased/template, kind-returning and unresolved type-family signatures remain runtime exports but are omitted from declarations; an unmapped concrete type reports the def and writes neither file. No any approximation.

Both generated paths use the existing input/directory guard before either is written. The separate existing hard-link issue #1495 / PR #1496 is not claimed fixed here.

Real behavior proof

Linux x86_64; Bun CLI 1.3.13, Node v22.23.1, TypeScript 5.9.3 installed as isolated verification tooling, not a repository dependency.

Before, the real typed consumer failed with exit 2 / TS7016 (no declaration for core.mjs). After actual CLI emission:

bun bend2/main.ts core.bend -o core.mjs
node path/to/typescript/bin/tsc --strict --module nodenext --target esnext good.mts
node good.mjs

Four actual TypeScript-compiled Node hosts passed:

• Slot/List consumer: true 9.
• Recursive Tree, callback and full/partial/zero-prefix calls: 10 10 9 9 9 9.
• Reserved type name class and helper-like type BendFn: 7 8.
• Native array update: [8,5].

Negative TypeScript consumers are rejected with TS2614 (invented named value export), TS2345 (invalid direct/partial call), TS2322 (malformed constructor field) and TS2339 (intentionally omitted erased signature). A concrete indexed type not mapped by this emitter exits 1 naming identity; neither output is created.

Regression checks

• Permanent tests/io/module_declaration_safety.bend and its real JS effect invoke the actual CLI against a derived-sidecar input symlink and directory. Before: prints 1; after: 0, source preserved and no partial module output.
• 20 real existing module/FFI/closure/marshal CLI cases matched their checked-in stdout and exit statuses.
• bun gates/repo.ts: PASS: 54 / 54.
• README updated. No parser/theory/checker, GUIDE, CHANGELOG, gate or cap edits.

Limits

No remote mini-cluster/macOS execution. Full dependent type semantics cannot be represented by TypeScript; only faithfully mapped signatures are declared, following the omission/error policy above. Static host checks were exercised with TypeScript 5.9.3.