Lint: enable stricter Biome rules and fix surfaced issues

[ryukzak]

Tightens the Biome ruleset beyond recommended to ratchet code quality, plus fixes the issues those rules surfaced.

Tier 1 — new rules at error (0 violations, pure ratchet)

• correctness: noUnusedVariables, noUnusedFunctionParameters
• complexity: noUselessFragments
• style: useImportType, useExportType, useThrowOnlyError, useDefaultSwitchClause, noEnum, useFilenamingConvention

Tier 2 — new rules at warn + fixes

Added at warn level, then resolved each (one commit per rule):

• noFloatingPromises — fixed 6 unawaited async calls. These were real bugs: cleanup() in builder.ts raced against generators writing the output dir, and writeNdjson/writeJsonFiles let IntrospectionWriter.generate() resolve before files finished writing.
• noUselessElse — converted 7 else-after-return blocks to guard clauses (matches house style).
• noEmptyBlockStatements — documented the intentional empty catch in the mustache config loader.
• useTopLevelRegex — hoisted 7 hot-path regex literals to module scope; disabled the rule for test/examples (one-shot assertions get no runtime benefit). Python generator sites left as-is per scope.

noMisplacedAssertion remains at warn (31 test-only hits) for a follow-up cleanup.

Config diff (biome.json)

// before
"style": {
  "useConsistentArrayType": "error",
  "useArrayLiterals": "error",
  "noNestedTernary": "error"
}

// after
"style": {
  "useConsistentArrayType": "error",
  "useArrayLiterals": "error",
  "noNestedTernary": "error",
  "useImportType": "error",
  "useExportType": "error",
  "useThrowOnlyError": "error",
  "useDefaultSwitchClause": "error",
  "noEnum": "error",
  "useFilenamingConvention": "error",
  "noUselessElse": "warn"
},
"performance": { "useTopLevelRegex": "warn" },
"nursery": { "noFloatingPromises": "warn" }

Example before/after for the noFloatingPromises fix:

// before — generate() resolves before files are written
this.writeNdjson(tsIndex.schemas, this.opts.typeSchemas, typeSchemaToJson);

// after
await this.writeNdjson(tsIndex.schemas, this.opts.typeSchemas, typeSchemaToJson);