instrument: HTTP/OTLP transport sinks + extras matrix CI

[mmercuri]

Summary

Lands the telemetry transport layer that adapter EventSinks ship
to, plus the CI gate that smoke-tests every adapter extra at both the
floor and ceiling of its declared pin range.

Scope

• src/layerlens/instrument/transport/sink_http.py — HTTPSink,
  async batching transport for the LayerLens ingest endpoint
• src/layerlens/instrument/transport/sink_otlp.py — OTLPHttpSink,
  hand-rolled protobuf marshalling against opentelemetry-proto so
  consumers don't pull in opentelemetry-sdk just to ship spans
• tests/instrument/transport/test_sink_otlp.py — 27 unit tests for
  the OTLP serializer + transport
• tests/instrument/test_sink_http_e2e.py — 9 end-to-end tests
  against a local FastAPI receiver
• samples/instrument/otlp_collector/ — runnable sample pointing the
  OTLPHttpSink at a Collector
• docs/adapters/otlp.md — configuration guide
• .github/workflows/extras-matrix.yaml — per-extra pin-range matrix
  that installs every framework / protocol / provider extra at both
  its floor and ceiling and runs an import-only smoke check
• pyproject.toml — two new optional extras
  • otel — opentelemetry-api + opentelemetry-sdk +
    opentelemetry-exporter-otlp-proto-http (for callers who want
    the full SDK)
  • otlp — opentelemetry-proto only (for the lean
    OTLPHttpSink path)

Blast radius

• Default pip install layerlens install set is unchanged. Both
  transport sinks are inert until an adapter is wired to them.
• No changes to existing public API surface.

Test plan

• uv run pytest tests/instrument/transport/ tests/instrument/test_sink_http_e2e.py -x
  — 38 passed
• Extras-matrix workflow exercises every adapter extra on PR

Stacks on

• feat/instrument-base-foundation (M1.A) — required for the
  EventSink contract this PR implements.

Linear

LAY-3400 umbrella (M1 transport tier). Unblocks Track A's
LAY-3401–3404 (compat / transport / CI work).

[mmercuri]

Linear: https://linear.app/layerlens/issue/LAY-3403 (OTLP HTTP transport sink + extras-matrix CI workflow — partial LAY-3404). Stacked on PR #93. Under Apollo M1 epic LAY-3423.