feat(telemetry): add OTLP export writer

[EhabY]

Summary

• add OTLP/JSON mapping for telemetry logs, spans, and metric-shaped events
• add .otlp.zip export writing with POST-ready logs.json, traces.json, and metrics.json
• extend writer tests to cover signal routing and OTLP zip contents

Refs #903.

Stack: 3 / 4. Base: #960. Next: #953.

Review follow-up

• exports spans as internal spans instead of unspecified spans
• includes window start timestamps for HTTP percentile gauges
• closes partially opened OTLP writers when setup fails
• drains queued zip writes before closing file handles on error paths

Validation

• pnpm test:extension ./test/unit/telemetry/export/range.test.ts ./test/unit/telemetry/export/files.test.ts ./test/unit/telemetry/export/writers.test.ts
• pnpm typecheck
• pnpm lint
• pnpm format:check

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