## Why
Codex telemetry pipeline needs a per-request TTFT value. The existing
`codex.turn_ttft` is recorded once per turn, so it cannot represent
later inference requests in the same turn and can miss the beginning of
hidden reasoning.
This restores the low-volume per-request signal proposed in
https://github.com/bk-nvidia/codex/pull/3 without bringing back
per-WebSocket-event TRACE logging.
## What changed
- start a timer when each mapped Responses stream begins
- latch the timer on the first `response.output_item.added`, including
an empty hidden-reasoning item
- attach `ttft_ms` to the existing `codex.sse_event` /
`response.completed` telemetry record
- cover the new completion field with an integration test
## Semantics
The value is per inference request, not per turn. It measures
mapped-stream-to-first-output-item latency, matching the
customer-proposed metric. For HTTP, the stream is already established
before timing begins, so request setup and response-header latency are
excluded.
`response.output_item.added` is a client-visible proxy for the start of
hidden reasoning; this does not claim access to the server's internal
first raw-token timestamp.
## Validation
- `just test -p codex-otel` (47 passed)
- `just test -p codex-core process_sse_emits_completed_telemetry` (1
passed after the final timer-placement change)
- attempted `just test -p codex-core`: 2,855 passed and 53 failed
because of unrelated local-environment failures (missing
`test_stdio_server` fixture binary, shell startup noise, and
timing-sensitive tests); the focused telemetry test passed in that run
as well
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