refactor(telemetry): tighten OTLP export writer and tests

- Restructure: writers.ts → writers/json.ts; combine OTLP mapping and
  writer into writers/otlp.ts; lift signal classification into
  export/signal.ts.
- Group all records of each signal under a single Resource block per
  file (was one Resource per event), matching the OTLP spec's
  recommended shape and shrinking exports at scale.
- Adopt OTLP enums from @opentelemetry/api + api-logs; document the
  +1 wire offset on SpanKind.
- Tighten the public surface: only writeOtlpZipExport and
  OtlpExportCounts are exported; record mappers are module-private.
- Hoist per-event metric attributes once; single-pass partition of
  http.requests measurements into counts and gauges.
- Share parseTelemetryTimestampMs between range.ts and the OTLP
  writer's nanos conversion (was duplicated with identical error).
- Extract asyncIterable test helper to test/mocks/; inline the memfs
  vi.mock pair across affected test files.
- Tests now exercise the public API only, dropping side-effect
  assertions about staging cleanup and atomic semantics already
  covered by writeAtomically's own tests.

Author: EhabY

package.json

@@ -675,6 +675,8 @@
 	],
 	"dependencies": {
 		"@abraham/reflection": "^0.13.0",
+		"@opentelemetry/api": "^1.9.1",
+		"@opentelemetry/api-logs": "^0.218.0",
 		"@peculiar/x509": "^2.0.0",
 		"@repo/shared": "workspace:*",
 		"axios": "^1.16.1",