fix: preserve FIFO order in overflow buffer

[1m55555555]

Summary

• Preserve FIFO ordering when the overflow buffer cannot immediately hand a task to a worker.
• Remove the oldest buffered task only after the handoff succeeds.
• Add a regression test covering failed forwarding with an existing backlog.

Problem

When the handoff channel was full, PushAndForward removed the oldest task before attempting to forward it. If forwarding failed, it appended that task back to the tail of the buffer.

This allowed newer tasks to overtake older ones, violating FIFO semantics and potentially delaying old tasks indefinitely during sustained backpressure.

Solution

Peek at the oldest task first, attempt the handoff, and remove it from the buffer only when the handoff succeeds. If it fails, the task remains at the head unchanged.

Validation

• go test ./...
• go test -race -count=1 . -timeout 5m
• go vet ./...

Scope

No public API changes. No changes to worker scaling, capacity limits, or task execution behavior.