Plan M4b: approvals over Mattermost in 15 tasks

Skeletons for new files only; existing files change by exact edits, so no task overwrites an
earlier one's work. Each task's tests were run against a reference built in order from master,
with the gate at each step and mutations checked; each skeleton compiles and fails its tests, and
every todo!() comment was read beside its signature. The reference is kept off this machine.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M4b/12-gatewayd-state-approvals.md

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+# M4b task 12: approval posts in the state file
+
+**Branch:** `m4b` (run `git switch m4b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `gatewayd: state, the posts of open approvals, and the turn of a session`
+
+## Goal
+
+Two things the serve loop needs from `state.json` (M4b spec, sections 6 and 8):
+
+- **Approval posts.** The posts of each approval shown and not yet ended, so that at the next
+  start the ones left behind can be marked dead. The field is **optional**: an M4a state file
+  loads unchanged. Like every other field, its ids must be valid, or the file is refused.
+- **The turn of a session**: where an approval of that session goes (its thread).
+
+## Files
+
+- Copy: `crates/gatewayd/tests/state_approvals.rs`
+- Modify: `crates/gatewayd/src/state.rs`, `docs/implementer-log.md`
+
+## The changes, exactly
+
+In `crates/gatewayd/src/state.rs`:
+
+**1.** In `struct StateFile`, after `in_flight: Vec<InFlight>,`:
+
+```rust
+    /// Optional, so an M4a state file loads unchanged.
+    #[serde(default)]
+    approvals: Vec<ApprovalPosts>,
+```
+
+and after `struct StateFile`:
+
+```rust
+/// The posts of an approval shown in Mattermost and not yet ended (M4b spec, section 8).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ApprovalPosts {
+    pub approval: u64,
+    pub channel: String,
+    /// First part first; the last is the one that is edited when it ends.
+    pub posts: Vec<String>,
+}
+```
+
+**2.** In `StateFile::problem`, after the `in_flight` loop and before `None`: each entry of
+`approvals` must have a `valid_id` channel and at least one post, all `valid_id`; otherwise
+`Some(format!("approval {}'s posts are not valid", shown.approval))`.
+
+**3.** Four methods at the end of `impl State`, each saving (`self.save()`) when it changes
+something, like the methods already there:
+
+- `pub fn turn_of(&self, session: &str) -> Option<InFlight>`: the entry of `in_flight` with that
+  session, cloned.
+- `pub fn add_approval(&mut self, shown: ApprovalPosts) -> Result<(), StateError>`: remove any
+  entry with the same `approval`, push this one, save.
+- `pub fn remove_approval(&mut self, approval: u64) -> Result<(), StateError>`: remove the entries
+  with that `approval`; save only if one was removed.
+- `pub fn take_approvals(&mut self) -> Result<Vec<ApprovalPosts>, StateError>`: take them all out
+  (`std::mem::take`); save only when there were some; return them. (As `take_in_flight` does.)
+
+## Steps
+
+- [ ] **1. Copy.** `git switch m4b`, then `cp docs/plans/M4b/files/crates/gatewayd/tests/state_approvals.rs crates/gatewayd/tests/`
+- [ ] **2. See it fail.** `cargo test -p gatewayd --test state_approvals`. Expected: it does not
+  compile.
+- [ ] **3. Make the three changes.** Run `cargo fmt --all`.
+- [ ] **4. See it pass.** `cargo test -p gatewayd --test state_approvals --test state --test serve_restart`.
+  Expected: 4, 4 and 7 passed.
+- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
+- [ ] **6. Log and commit.** `git add crates/gatewayd docs/implementer-log.md Cargo.lock && git commit`
+
+## Done when
+
+- The three suites pass; `make gate` prints `gate: ok`.