bedrock-kv / bedrock / b211a889780e5f478e8c33737f8353c293a193dc

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Adopt bedrock_raft 0.10 and flatbuffer 0.6; extend DiskRaftLog to the new Log contract (#230)

Re-opens the dependency stack against `develop`. #225 was closed
unmerged and #226 was then merged *into* this branch, so the whole stack
has been sitting off `develop` since 2026-09-03 — `develop` still pins
`{:bedrock_raft, "~> 0.9"}` / `{:flatbuffer, "~> 0.5"}`. This PR lands
it. Closes bedrock-54w and bedrock-fu8.

Two commits: a mix.lock refresh, and the raft/flatbuffer major bumps
with the code they require.

## Why

`bedrock_raft` 0.10.0 extends the `Bedrock.Raft.Log` protocol and
changes the shape of the ack telemetry event, so the coordinator's
`DiskRaftLog` no longer satisfies the contract it implements. Three of
the additions exist for real reasons we want:

- **Votes must survive a crash.** Pre-0.10 the log persisted only
`current_term`; a node that voted, crashed, and came back could vote
again in the same term. 0.10 makes `voted_for/1` +
`save_election_state/3` part of the protocol so term and vote are
written together.
- **Replication is batched.** `transactions_from/4` lets the leader pull
one bounded batch per AppendEntries instead of everything from the
follower's cursor to the tail.
- **Purges must be honest.** The leader now repositions its send cursor
from a follower's hint via `has_transaction_id?/2`, so the log can't
leave purged records physically present.

Separately, `flatbuffer` 0.6 moves to compile-time, schema-specialized
codegen. 0.6.1 is what makes it adoptable cleanly (see below), and with
it every dep resolves from hex — the interim git-tag dep on
`bedrock_raft` is gone (`~> 0.10`), so nothing blocks `mix hex.publish`.

| dep | from | to |
|---|---|---|
| bedrock_raft | 0.9.7 | 0.10.0 |
| flatbuffer | 0.5.0 | 0.6.1 |
| iodata | 0.7.3 | 0.9.0 |
| telemetry | 1.3.0 | 1.4.2 (unblocked by raft relaxing its pin to `~>
1.3`) |

Plus a routine lock refresh of dev/test tooling with no `mix.exs`
requirement changes: req 0.7.4, mox 1.3... (continued)

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