Add M2a plan: thirteen tasks, tests, fake server and recordings

The tasks build the inference path: emsha-backed SHA-256, inferproxy,
config, a hand-written HTTP and SSE client, request building, delta
assembly, the chat state machine, the thinking cap, the slot gate with
retry, the startup self-test and on-device verification.

Everything the tasks copy in was checked against a private reference
implementation: the gate passes after each task in order, the timing
tests pass repeatedly under CPU load, and the reference passes the
self-test and all four device checks on straylight. Expected results
for the recorded streams were derived by a separate script.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/plans/M2a/05-loopd-sse.md

@@ -0,0 +1,86 @@
+# M2a task 05: the SSE reader
+
+**Branch:** `m2a` (run `git switch m2a`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `Add a server-sent-events reader to loopd`
+
+## Goal
+
+Turn a streamed body into a sequence of `data:` payloads. `llama-server` streams a completion as
+server-sent events: one JSON object per `data:` line, and `data: [DONE]` at the end.
+
+## Context
+
+```
+data: {"choices":[…]}\n
+\n
+: a comment line\n
+data: {"choices":[…]}\r\n
+\r\n
+data: [DONE]\n
+\n
+```
+
+Lines end in `\n` or `\r\n`. Blank lines, comment lines (starting with `:`) and any field other
+than `data` (`event:`, `id:`, …) carry nothing for us and are skipped. After `data:` one leading
+space is dropped if present, and only one.
+
+## Files
+
+- Copy: `crates/loopd/tests/sse.rs`
+- Create: `crates/loopd/src/sse.rs`
+- Modify: `crates/loopd/src/lib.rs`, `docs/implementer-log.md`
+
+## Interfaces
+
+Consumes: `loopd::http::is_timeout`.
+
+Produces, in `crates/loopd/src/sse.rs`:
+
+```rust
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum SseItem { Data(String), Done }     // Data: the text after "data:", without the line ending
+
+#[derive(Debug)]
+pub enum SseError { Io(std::io::Error), Timeout, Truncated, LineTooLong, NotUtf8 }   // Display, Error
+
+pub struct Events<R> { /* private */ }
+impl<R: std::io::Read> Events<R> {
+    pub fn new(reader: R, max_line: usize) -> Self;
+    /// The next item, or None when the stream has ended cleanly between lines.
+    pub fn next_item(&mut self) -> Result<Option<SseItem>, SseError>;
+}
+```
+
+Rules the tests check:
+
+1. The result must not depend on how the bytes arrive. A reader that gives one byte per call must
+   produce the same items, including for text with multi-byte characters: decode a line as UTF-8
+   only once the whole line is there.
+2. A line longer than `max_line` bytes is `LineTooLong`. This applies **both** when the line is
+   still arriving with no newline in sight **and** when it arrives whole in a single read. Check
+   the length in both places.
+3. When the reader ends (`read` returns 0): with nothing buffered, return `Ok(None)`, and keep
+   returning it if asked again. With a partial line buffered: `Truncated`.
+4. Reader errors: `is_timeout` gives `Timeout`; `UnexpectedEof` gives `Truncated` (the HTTP body
+   reader reports a cut stream that way); `Interrupted` is retried; anything else is `Io`.
+5. A line that is not valid UTF-8 is `NotUtf8`.
+
+## Steps
+
+- [ ] **1. Copy.** `git switch m2a`, then
+  `cp docs/plans/M2a/files/crates/loopd/tests/sse.rs crates/loopd/tests/`
+- [ ] **2. See the test fail.** `cargo test -p loopd --test sse`. Expected: it does not compile.
+- [ ] **3. Write `sse.rs`,** and add `pub mod sse;` to `lib.rs`. Run `cargo fmt --all`.
+- [ ] **4. See the test pass.** `cargo test -p loopd --test sse`. Expected: `7 passed`. The last test
+  reads four recordings through the HTTP client from task 04.
+- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
+- [ ] **6. Log and commit.** `git add crates/loopd docs/implementer-log.md && git commit`
+
+## Done when
+
+- `cargo test -p loopd --test sse` reports 7 passed; `make gate` prints `gate: ok`.
+
+## Stop and report if
+
+- `recorded_streams_have_the_expected_shape` fails although `cargo test -p loopd --test http`
+  passes and the other six tests here pass.