M4a tasks 14 and 15: glue written, calls tabled, after task 14's session wrote nothing

Task 14's session read the crate to learn the APIs and planned `connect` in prose until it was
cut off. `connect`, `Gateway::new` and `catch_up` are now written; the task lists every call with
its signature and makes the copy and the failing test the first actions. Task 15's comment is
rewritten one step per item. Both checked to fail, then pass when filled from their comments.

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

docs/plans/M4a/files/crates/gatewayd/src/main.rs

@@ -25,13 +25,20 @@ fn main() -> ExitCode {
 }
 
 fn serve(path: &Path) -> ExitCode {
-    // Each failure prints one line (plus its pointer) and returns ExitCode::from(1):
-    // 1. `Config::load`: "gatewayd: <error>\n<START_FAILED>". 2. `token_source`: "gatewayd: <path>:
-    //    <why>\n<START_FAILED>".
-    // 3. `secrets::load(MATTERMOST_TOKEN, &source, &|k| std::env::var_os(k))`: "gatewayd: <error>"
-    //    (it carries its pointer). Print the warning, if any, as it is. 4. Create the state file
-    //    directory, recursive, 0700: "gatewayd: cannot prepare <dir>: <e>\n<START_FAILED>". 5.
-    //    `run` with Tuning::default(), a log that prints each line to standard error, and a stop
-    //    flag that is never set; print the Stop it returns.
+    // Each failure prints its message with `eprintln!` and returns `ExitCode::from(1)`.
+    // 1. `let config = match Config::load(path) { ... }`: an Err(e) prints
+    //    "gatewayd: {e}\n{START_FAILED}".
+    // 2. `let source = match config.token_source() { ... }`: an Err(why) prints
+    //    "gatewayd: {}: {why}\n{START_FAILED}" with `path.display()`.
+    // 3. `let loaded = match secrets::load(MATTERMOST_TOKEN, &source, &|k| std::env::var_os(k))`:
+    //    an Err(e) prints "gatewayd: {e}" (the error carries its own pointer).
+    //    Then `if let Some(warning) = &loaded.warning { eprintln!("{warning}"); }`.
+    // 4. The state file's directory:
+    //    `let dir = config.state_path().parent().map(Path::to_path_buf).unwrap_or_default();`
+    //    `std::fs::DirBuilder::new().recursive(true).mode(0o700).create(&dir)`: an Err(e) prints
+    //    "gatewayd: cannot prepare {}: {e}\n{START_FAILED}" with `dir.display()`.
+    // 5. `let log: gatewayd::serve::Log = Arc::new(|line: &str| eprintln!("{line}"));`
+    //    `let stop = run(config, loaded.secret, Tuning::default(), log, &AtomicBool::new(false));`
+    //    `eprintln!("{stop}");` and `ExitCode::from(1)`.
     todo!()
 }