Coordinate measurement batches with a readiness barrier

[ctfbruce]

Measurement batches can now exchange peer metadata and wait for a shared future start after every admitted run is ready. Guests use debuglet.Ready and debuglet.WaitStart; the dispatcher persists a bounded, one-shot barrier under the existing batch and run identities. The executor uses its authenticated control connection, and existing network policies still govern peer traffic.

The client helper and manifest runner preserve each participant's program hash, arguments, policy and run ID, with grouped result export and cancellation. A five-executor UDP example exchanges traffic directly between peers and records requested and observed timing. Readiness is limited to 30 seconds and 4 KiB of metadata per participant; simultaneous execution, clock synchronization, NAT traversal and packet retries are outside this slice. Dispatcher schema 28 adds the persisted barrier and readiness records; guests require the optional debuglet_experiment_v1.ready extension.

Validation: final CI passed all 16 jobs, including the full test/race suites and installed compatibility. The earlier installed five-executor rehearsal passed in 4.10 seconds: each participant sent and received four datagrams, shared one experiment/start value, and exported grouped results successfully. Observed starts were 0.77–3.20 ms after the requested time on one Linux runner; this is local evidence, not a cross-host synchronization guarantee.

Refs #7 (extends the existing structured rendezvous support to fixed batches).