fix(opencode): count silent streamed replies while the first-byte guard stays off

[maxmad64bis]

⚠️ base-red inherited: #15306

Summary

Streamed replies that go silent after headers pass through without a trace, so the 15 s first-byte window cannot be calibrated. The change adds a passive shadow watch: when the guard stays off, a silent streamed reply is now counted and logged with its running total, while the reply itself is returned untouched. Talking streams, non-streamed calls and other formats stay as before, and nothing rotates, cools down or changes defaults.

Related Issues

• Related to fix(proxies): steer selector to members without recent refusals #14804

Validation

• Change type: other
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/opencode-responses-stall-measure.test.ts — new: silent reply counted and logged with the reply returned intact; talking body stays silent; non-streamed and non-Responses requests ignored; armed guard excludes the shadow; storage stays constant.
• tests/unit/opencode-responses-stall-harden.test.ts — untouched, rerun green; covers the unchanged guard.
• tests/unit/opencode-responses-first-byte-stall.test.ts — untouched, rerun green; covers the executor wiring.

Coverage Notes

• open-sse/executors/opencodeResponsesStall.ts — covered by the new measure test (shadow counting, exclusion paths, constant storage) plus the harden suite for the unchanged guard.
• open-sse/executors/opencode.ts — one wiring hunk, exercised through the executor-level first-byte stall suite.

Reviewer Notes

• Behavior: guard off plus a silent streamed reply now counts and logs the occurrence (count, total silent time, window) while the reply passes through untouched; no rotation, no cooldown, no default changed.
• Merge: shares the executor wiring file with fix(sse): name the egress on every opencode rotation log line #15375, fix(proxies): set aside region-refused members briefly #15383 and fix(opencode): only a refusal naming the model clears borrowed tools #15475 — either order, with a rebase on the release tip.
• Follow-up: replay the neighbor search on opening day and read the owner position. Rebase after feat(proxies): accept operator-provided dated egress observations for pool members #15314, feat(api): add /v1/me/sessions/{id}/messages endpoint with opt-in turn capture #14864, feat(api-keys): opt-in flag to count unpriced usage as $0 in per-key USD quotas #14799 and feat(api): configurable unpriced-usage budget policy + admin notice (#14604) #14608 if they land first. Revisit the four open review notes (floating clone cancel, debug line on shadow misses, untested zero-window fallback, promise-typed wrapper). Exploit the shadow counts on live Responses traffic, then confirm or revise the 15 s first-byte window.