api: NewBatchRunner — the batch fetch adapter, retry on busy, outcome metrics

Wire the batch runner to fetchTarget and feed the sift_batch_* metrics.
New internal/api/batch.go: NewBatchRunner overwrites cfg.Hooks (Job, Target,
JobSeconds, Depth, Inflight → s.Prom) and sets the runner's FetchFunc to
batchFetch, which tags the context as a batch fetch, runs under requestDeadline,
and maps fetchTarget's errors to what the runner retries (429/503 →
ErrRetryLater) and what the metrics classify (batchOutcome). Added the Batch
field to Server (nil → task 07 answers 503 "batch disabled") and, in
extractPDF, charged the PDF pool to c.Name+"/batch" under the batch mark so a
batch never takes a caller's interactive slot while the single-flight key is
unchanged. No HTTP endpoints.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

docs/implementer-log.md

@@ -22,6 +22,7 @@ One row per task session that ends in a commit. Newest last.
 | p6/03-runner-workers | 2026-09-26 | done | 1 | gate: ok | none | Workers in new internal/batch/worker.go: one rotation of live jobs in submit order, round-robin via an absolute cursor (advances in pick, wraps at access, so it survives a second job being added while the first is in flight — the mod-n cursor from the design stuck at 0 for one job and re-picked it), serial per job (target i+1 never starts before i is stored), one target per fetch under the Close-cancelled context (panic→ErrInternal, never echoed; ErrRetryLater retried after RetryWait, bounded by RetryFor then stored failed with the wrapped text). Terminal status once per job: all done→done, all failed→failed, mix→partial. Cancel of a running job sets cancelled, stores the in-flight target without overwriting cancelled, leaves later targets pending. Close sets closed, cancels the context, broadcasts, waits the workers, idempotent. batch.go: New starts cfg.Workers, Submit appends the liveJob and wakes, Cancel handles the in-flight case, Close stops. Absolute cursor is the one deviation from the design's "index in live" comment. All race tests pass -count=3. | |
 | p6/04-metrics | 2026-09-26 | done | 1 | gate: ok | none | Batch series in new internal/metrics/batch.go, registered in New: `BatchJobsTotal`/`BatchTargetsTotal` CounterVecs (label "outcome", any value outside the known set folded to "other" via `batchJobOutcomes`/`batchTargetOutcomes` maps so a raw string never becomes a label), `BatchQueueDepth`/`BatchTargetsInflight` gauges, and `BatchJobSeconds`/`BatchTargetSeconds` histograms (durationBuckets) plus `BatchResultBytes` (1K,4K,16K,64K,256K,1M). Nil-receiver-safe `Observe…`/`Set…` methods mirroring ObservePDF/ObserveDigest. Series served through the existing /metrics handler; no new endpoint, no Snapshot change. Given test byte-identical; metrics tests pass under -race -count=3; make gate ok. |
 | p6/05-fetchtarget | 2026-09-26 | done | 1 | gate: ok | none | Extracted v1fetch's per-target pipeline into new `internal/api/target.go`: `fetchTarget(ctx, c, t batch.Target) (batch.Result, error)`. Body moved unchanged in order — question truncation to 500 runes, mode default/check, wantCtx, doFetch, recover path, PDF branch (render, pages/grep errors, question-chunk limit), clean/raw branch, digest, context search, the fetch log line, and the envelope. `content` is returned as `Result.Content`; every other envelope key (`mode`,`untrusted`,`source`,`warnings`,`sizes`,`timings`, and `pdf`/`digest`/`context` when present) goes in `Result.Meta`, never content. `v1fetch` keeps reqN/caller/decode/400, builds a `batch.Target` (`URL` + `Mode`/`Question`/`Pages`/`Grep`/`JQ`/`Context`/`Recover` into `Options`), calls `fetchTarget`, `writeFetchErr` on error and `writeJSON(w,200,env)` on success where `env` is a fresh copy of `Meta` plus `"content"` (a copy, so `len(env)==len(Meta)+1` holds and Meta is not mutated). The two `writeErr` failures become `*fetch.Error` with the same status/msg/hint: 400 "invalid url" (bad mode), 400 "raw mode does not apply to PDFs". `doFetch` takes `context.Context` instead of `*http.Request` (its three `r.Context()` → `ctx`); `scrape` passes `r.Context()`. Removed now-unused `fmt`/`unicode/utf8`/`recover` imports, added `batch`. All api tests pass under -race -count=3; make gate ok; given test byte-identical. |
+| p6/06-batch-wiring | 2026-09-26 | done | 1 | gate: ok | none | New `internal/api/batch.go`: `NewBatchRunner` overwrites `cfg.Hooks` so Job→ObserveBatchJob, Target→ObserveBatchTarget(batchOutcome)+ObserveBatchTargetSeconds (+ObserveBatchResultBytes only when err==nil), and JobSeconds/Depth/Inflight→the matching `s.Prom` methods; the runner's FetchFunc is `batchFetch`, which tags ctx with an unexported `batchCtxKey`, runs under `requestDeadline`, and maps fetchTarget's errors (a *fetch.Error 429/503 → `fmt.Errorf("%s: %w", fe.Msg, batch.ErrRetryLater)` so the runner retries; any other *fetch.Error → TargetError with 502 rewritten to 500 "internal error" and `s.blockN.Add(1)` for 403; anything else → TargetError 500 logged Error with `s.logErr`). `batchOutcome` classifies nil→ok, ErrRetryLater-wrapped→busy, then by TargetError.Status (403 blocked, 429 busy, 504 timeout, 415 unsupported, 422 whose Msg starts "document too long"→too_long, other 4xx→upstream_4xx, 5xx→upstream_5xx, any other error→upstream_5xx). Added `Batch *batch.Runner` Server field (nil → the /v1/batch endpoints answer 503 "batch disabled"). `pdf.go` extractPDF charges the PDF pool to `c.Name+"/batch"` when the ctx carries the batch mark, single-flight key unchanged so a batch and an interactive extract of the same PDF still share one child; the only new log line carries caller and host="-" (never a URL/host). All api tests pass under -race -count=3; make gate ok; given file byte-identical. |
 
 ## P5a review (owner, 2026-09-26)