refactor!: explicit pipeline abstractions and typed JS surface

[Andrzej-Swietek]

Summary

The generator was correct but hard to change safely: no traits, diagnostics
threaded as &mut through 22 signatures, schema depth counted by convention
at each call site, four files past 400 lines. Two checks were wrong: a
repeated key under paths reported as a duplicate schema name, and the
fixture list regen-snapshots read had drifted from test/fixtures/.
Nothing type-checked the published JavaScript or the tests.

How to review

About 60% of the diff is mechanical. Skim: the src/ir → src/api_model
rename, module splits (operations.rs, services.rs, request.rs,
options.rs, typescript.rs), doc-comment trimming, scripts/*.mjs →
*.ts, JSDoc in lib/ and bin/, and the 26 added snapshot files.

Read, in pipeline order:

File	What changed
src/error.rs	Reporter moves to RefCell; every stage takes &Reporter
src/parse/unique_map.rs, src/parse/input.rs	duplicate keys rejected at decode; duplicate-schema-name routes on the exact components.schemas: prefix
src/api_model/normalize/walk.rs	SchemaWalk replaces (context, depth, reporter) triples
src/subcode.rs	closed subcode set, tested against index.d.ts.in
src/plan/artifact_plan/resolve.rs	rejects group/method names that yield an empty file stem
src/emit/ts/{writer,types}.rs	Writer, Render, Position
lib/wrapper-core.js, scripts/patch-types.ts	InputFormat/ResponseType become runtime constants
scripts/lib/snapshot-layout.ts	single fixture manifest; check-fixture-coverage gates CI

Behaviour changes

Generated output is unchanged. These are not covered by that claim:

• A repeated key under paths now fails as E_INPUT_INVALID, not
  E_POLICY_VIOLATION — caught while decoding, before the policy pass.
• A JSON input with a repeated key was last-wins and now fails. serde_json
  carries no field path, so in JSON a duplicate schema name gets no
  duplicate-schema-name subcode; YAML still routes it.
• duplicate-schema-name now matches components.schemas: exactly, so a
  nested path like Pet.properties is a plain decode failure.
• The anchor guard no longer passes a source whose Value parse failed on a
  duplicate key.
• A group name with no letters or digits is rejected; it produced
  rest/.rest.ts and rest//index.ts.
• InputFormat and ResponseType stop being const enum and are exported
  as runtime constants from both entries.
• build:debug now runs postbuild, so a debug build leaves a clean tree.

Verified

• 325 Rust tests, 400 ava tests; clippy --all-targets, fmt, oxlint and
  three typecheck projects clean.
• 131 pre-existing snapshots unchanged; 26 snapshot files added for ten
  newly pinned fixtures plus bench-multi-tag.
• templates/ byte-identical to main. native.js differs only in the
  napi version-check strings (0.4.0 → 0.6.0, matching package.json);
  browser.d.ts exports the two new runtime constants.
• CLI output identical across 21 invocations against the base.
• YAML throughput 1.6–2.2x, JSON unchanged — the win is the removed double
  parse.

Follow-ups (not in this PR)

• rest-util-operations.spec.ts casts bound.request to a two-argument
  signature while templates/angular/rest.util.ts declares it with none.
  Predates this PR (feat!: standalone operations layout, drop the .generated suffix #8); to be settled in its own issue.
• Three comments and website/README.md still name bundle-engine.mjs.

[pkurcx]

Thanks, this is a careful refactor and the structure is a clear improvement. I built both this head and its base commit (1f1944e) and ran every fixture in test/fixtures through both CLIs: generated files, stdout, stderr and exit codes are identical for all 73, except the disclosed duplicate-schema-name wording. That covers the ten newly pinned fixtures too, which the new snapshots alone cannot prove. Also confirmed: fmt/clippy/cargo test clean (305), typecheck/lint/coverage/ava clean (345), and all ten commits compile standalone.

Two things need fixing before merge, and one rebase hazard:

1. Runtime enums are declared but not exported. index.d.ts now promises InputFormat and ResponseType as runtime constants; lib/index.js and lib/browser.js export only generate, GenerateError, EmitTarget. InputFormat.Yaml (shown in the node-api docs) type-checks and throws at runtime. Under the old const enum it was inlined and worked. Inline comment below.
2. Inline-object depth is charged twice per level in schema/mod.rs, roughly halving the nesting cap. Inline comment below.
3. Rebase onto main. main has moved (v0.6.0, #8): 20 files conflict. #8 also added a Layout const-enum rewrite to scripts/patch-types.mjs, which this PR deletes, so that rewrite has to be ported into patch-types.ts or it silently disappears. The ten new snapshot dirs need regenerating under the new model.ts / rest/<tag>.rest.ts names.

Smaller behaviour changes worth listing in the description, since they are not covered by the byte-identical claim:

• JSON inputs with a repeated key were last-wins on the base and now fail with E_INPUT_INVALID (no subcode, so a duplicate schema name in JSON never gets duplicate-schema-name).
• A repeated key under paths changes code from E_POLICY_VIOLATION to E_INPUT_INVALID, not only its message.
• default_group now returns Default for tags: [""] where the base fell through to the path segment.
• Re-export lines (export type { A as X, ... } from) now wrap at 100 columns.

Non-blocking: a few why-comments dropped in e60917d are worth restoring (the Params-suffix collision rationale in plan/naming/fixed.rs, the verbatim-brace-instead-of-panic note in emit/angular/request.rs, the alias-collision note in emit/model/emit_ts_models.rs). The Emitter trait has one impl per target and no second implementation planned, so a plain function would carry less indirection; fine to keep if you prefer the shape.