Add processor graceful shutdown

[karenychen]

Summary

• add Processor.Close(ctx) as terminal, idempotent shutdown for stopping receives and abandoning messages held by the processor
• keep inFlightMessages as a small mutex-protected message map; its close(ctx, settler) snapshots tracked messages under RLock, abandons each with a 10-second per-message context, and forgets each message only after abandon succeeds
• use an internal shutdown context to cancel Start and in-flight handler contexts without requiring the caller to cancel the original start context
• keep the processor semaphore for receive sizing for now, with a TODO to remove it in a later simplification
• pause receiving before abandoning by canceling shutdown and waiting for any active receive batch to finish registering messages
• add unit coverage for inFlightMessages, processor close behavior, the receive-before-close abandonment ordering, abandon timeout context, and keeping failed abandons tracked for retry

Validation

• go test -run 'TestInFlightMessages|TestProcessorStart|TestProcessorClose' .
• go test ./...
• go test -race -run 'TestInFlightMessages|TestProcessor' .
• git diff --check

Notes

• Full go test -race ./... still hits pre-existing lock renewer test issues in v2/lockrenewer_test.go; this is logged locally in .issues/019eadca-cd7a-7a63-81c4-ce36450690f1.md.