fix(schema): dedupe recursive JSON Schema $ref-to-$id types

[schani]

Bug

When a JSON Schema has a self-recursive $ref back to its own $id (or a
$ref: "#" self-reference), quicktype generated a duplicate, self-recursive
copy of the type — plus duplicated copies of every type nested underneath it.

Repro (--lang cs --src-lang schema --top-level Tree tree.schema.json with a
tree-node schema whose children property is $ref'd back to the schema's
own $id) produced four C# classes (Tree, a duplicate self-recursive
TreeSchema, TreeData, and a duplicate ChildData) instead of the expected
two (Tree, TreeData), with Tree correctly self-recursive. Same
duplication happened for every target language (also reproduced with
TypeScript), since the bug is in the shared JSON Schema input handling.

Root cause

In packages/quicktype-core/src/input/JSONSchemaInput.ts, Refs that
represent "the root of an addressed document" were constructed with two
different, non-equal path shapes depending on the code path:

• Ref.parseURI special-cased an empty fragment on an addressed URI to
  produce path [{kind: Root}].
• Ref.root(address) (used by Canonizer.addSchema and
  Resolver.resolveTopLevelRef) constructed a bare Ref with path [].

These two "same" refs weren't equal under the EqualityMap used to cache
already-converted types (typeForCanonicalRef), so when a self-$ref
resolved back to the document root, the cache missed and the schema got
converted a second time — only the second copy ended up wired up as truly
recursive, leaving the first copy (and everything nested under it) as a
duplicated dead end.

Fix

Normalize this invariant in one place — the Ref constructor itself: any
Ref with a defined addressURI and an empty path is normalized to path
[{kind: Root}]. This makes every construction path (Ref.root,
Ref.parseURI, and any other new Ref(address, [])) agree on a single
canonical representation for "root of an addressed document," so nested
paths built by pushing onto any of them stay comparable.

An initial version of this fix only changed Ref.parseURI's normalization
without touching the invariant globally, which fixed the reported bug but
broke two other things that depend on ref-path shape: the CLI's
properties-are-types feature for <file>#/-style source URIs, and
$id-based self-references combined with explicit #/...-style refs (the
existing id-root.schema fixture). Both are covered by the regression tests
added below and now pass together with the original fix.

Test coverage

• test/inputs/schema/recursive-ref-to-id.schema +
  test/inputs/schema/recursive-ref-to-id.1.json — new end-to-end fixture
  reproducing the reported bug (self-$ref to the schema's own $id),
  wired into the schema fixture registry (skipped for Elm/Dart, which don't
  support this kind of recursion at all, matching the pattern already used
  for other recursive schema fixtures).
• test/unit/recursive-ref-to-id.test.ts — unit test asserting exactly 2
  generated TypeScript interfaces (not 4) for both the $ref: "T2" and
  $ref: "#" self-reference forms, since fixture round-tripping alone
  doesn't catch "duplicate type" bloat.
• test/unit/schema-properties-as-types.test.ts — regression coverage for
  the <file>#/ properties-are-types CLI feature that an earlier version of
  this fix broke.

Verification

• npm run build passes.
• npm run test:unit — 27 files, 167 tests, all pass.
• Re-ran the original repro manually: now produces exactly Tree and Data
  (renderer-named nested type) in C#, with Tree.Children typed as Tree[]
  — no TreeSchema/ChildData duplicates.
• Re-ran FIXTURE=schema-typescript script/test across all
  test/inputs/schema/*.schema fixtures except vega-lite.schema,
  class-map-union.schema, and class-with-additional.schema (all three
  confirmed to already fail the same way on unfixed master — pre-existing,
  unrelated TypeScript tsc strictness issues) — all pass, including
  id-root.schema which the intermediate fix attempt had broken.
• C#-specific fixture compilation (dotnet) was not available in this
  sandbox, so the C#/schema-cs fixture wasn't run end-to-end; CI will
  validate it, and the shared input-layer fix plus the TypeScript fixture
  coverage above exercise the same code path.

Fixes #2836.

🤖 Generated with Claude Code

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5563 changed lines — +3191 / −2372

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[github-actions]

Generated-output differences

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5838 changed lines — +3308 / −2530

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