worker: write v2 envelopes with typed commands

[muhamadazmy]

worker: write v2 envelopes with typed commands

Switch the worker's write path from the v1 Envelope/Command enum to v2
typed commands and Envelope.

SelfProposer::{self_propose, self_propose_many, append_with_notification}
are now generic over C: Command + HasRecordKeys and derive the partition
key from the command via record_keys() — the explicit partition_key
parameter threaded through LeadershipState, the Actuator trait, and every
RPC handler is gone.

All worker RPC paths (append_invocation, append_invocation_response,
append_signal, cancel/kill/purge_invocation, purge_journal,
resume_invocation, restart_as_new_invocation, get_invocation_output) and
all leader-side ActionEffect proposals (Invoker, Shuffle, Timer, Cleaner,
UpsertSchema, UpsertRuleBook, scheduler decisions, version barriers,
AnnounceLeader) now construct typed commands::Command values directly,
removing the Command:: enum wrapping and the BytesMut arena previously
used to pre-encode scheduler decisions.

With v2 envelopes now being written, drop the try_v1_first branch in
PartitionProcessor::decode_record — v2 is the fast path and v1 stays as
the TypedValueMismatch fallback for replay of older records. Tests under
leadership and rpc are updated to mock the new generic Actuator signatures
and decode via Envelope::into_typed::commands::*().

[github-actions]

Test Results

  8 files  ±0    8 suites  ±0   4m 53s ⏱️ +5s
 50 tests ±0   50 ✅ ±0  0 💤 ±0  0 ❌ ±0 
218 runs  ±0  218 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 8032ac8. ± Comparison against base commit d0aa1be.

♻️ This comment has been updated with latest results.

[muhamadazmy]

This is preparation work for v1.8. Should not be merged now:

Note to self:

• kafka ingress should also start ingesting v2 envelopes.