fix(router): provisional cursor on BatchFlusher poison skip

[olucasandrade]

Source fix plan

.claude/fix-plans/router-poison-skip-batchflusher-cursor-20260924-212132.md

Problem

In applyDispatchCursors, the RTR-07 poison-skip branch called advanceCursorLocked, which advanced cursorByPartition and queued SaveCursor immediately. For BatchFlusher consumers, Deliver only buffers — the durable cursor must move only after FlushBatch succeeds via provisional promotion.

Trace: durable cursor at 1; poison at seq 3 → skip persisted cursor to 4 while seqs 1–2 were only buffered; flush failure discarded provisional → next read started at 4 → seqs 1–2 lost. Real queue sinks drop their pending buffer on flush failure, so this is an at-least-once violation.

Introduced by

36c9733

Implementation

• In applyDispatchCursors, BatchFlusher consumers record a poison skip on provisionalByPartition (same promotion path as a successful buffered Deliver).
• Non-flusher consumers keep immediate advanceCursorLocked.
• Skip-set still suppresses re-delivery of the poisoned seq after a failed flush re-reads the partition.
• CHK-01 and non-flusher cursor semantics unchanged.

Validation

• CGO_ENABLED=0 go test ./internal/router -count=1 → pass (18.5s)
• go test -race -count=1 -timeout 180s ./internal/router → pass (22.9s)
• New tests: mid-window poison (seq 3) + next flush failure re-delivers seqs 1–2; no cursor past unflushed buffered seqs while flush keeps failing; existing poison-at-start + successful flush tests remain green.

Residual risk

• Incorrect provisional bookkeeping could still re-deliver a poisoned seq or stall a key if skip-set and provisional promotion get out of sync under unusual concurrent partition patterns; covered for the mid-window + flush-failure path but not every multi-consumer floor interaction.

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