Gantry: 10 - Lifecycle-bound background work

[vpatelsj]

Scope

Bind best-effort background work to agent lifecycle.

Problem

firePrefetch and reAdvertiseDigest start goroutines with context.Background(), so shutdown cannot cancel them and manifest bursts can create unbounded concurrent best-effort work.

Evidence:

• internal/gantry/mirror/mirror.go:1756
• internal/gantry/mirror/mirror.go:1774

Changes

• Pass an agent-scoped context or a cancellable child into firePrefetch and reAdvertiseDigest instead of context.Background().
• Add a small semaphore if manifest bursts create too many concurrent goroutines.
• Ensure shutdown cancellation stops these best-effort goroutines before discovery/libp2p teardown completes.

Tests

• Lifecycle cancellation stops prefetch and readvertise goroutines.
• Shutdown does not start new background prefetch/readvertise work after drain begins.