perf(p2p): bound catalog lookup latency across multiple peers

[catyans]

Problem

IrohBlobsP2pTransport::lookup_peers currently queries candidate peers
sequentially. Each remote lookup has its own timeout (5 seconds by default), so
stale or unreachable peers at the front of the scheduler-provided list can make
catalog lookup latency grow linearly with the number of failed candidates.

The implementation already calls this out with:

// TODO: parallelize lookups to multiple peers.

This affects both scheduler-index hits and broad peer-discovery fallback.

Proposed change

Run remote catalog lookups with a small fixed concurrency bound while retaining
the scheduler-provided ordering when selecting a successful descriptor.

• Check local/self candidates without network I/O.
• Keep at most a bounded number of remote lookups in flight.
• Yield results in candidate order so a slower, higher-priority peer still wins
  over a lower-priority peer in the same window.
• Continue past misses, timeouts, and malformed/unreachable peers.
• Avoid adding a user-facing tuning knob until measurements show one is needed.

This reduces worst-case lookup time from approximately N * lookup_timeout to
ceil(N / concurrency) * lookup_timeout without generating an unbounded
connection burst or discarding scheduler priority.

Validation

• Unit tests for ordering, bounded concurrency, misses, and errors.
• Existing Iroh loopback lookup/fetch tests.
• Multi-node Lingjun experiment with failed candidates preceding a valid
  provider, comparing lookup latency before and after the change.