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/files/Makefile

@@ -0,0 +1,28 @@
+# Boxmaker gate. `make gate` must pass before any work is called done. It needs no network.
+
+.PHONY: gate audit verify-device
+
+gate:
+	cargo fmt --all --check
+	cargo clippy --workspace --all-targets --locked --offline -- -D warnings
+	cargo test --workspace --locked --offline
+	cargo deny --offline check bans licenses sources
+	sh scripts/check-lines.sh
+	sh scripts/check-crate-deps.sh
+	sh scripts/check-dep-docs.sh
+	sh scripts/test-gate-scripts.sh
+	@echo "gate: ok"
+
+# Fetches the RustSec advisory database. Listed in docs/egress.md.
+audit:
+	cargo deny check advisories
+
+# Checks that need straylight. They go through a private inferproxy to the real server.
+# Override the server with: make verify-device UPSTREAM=host:port
+UPSTREAM ?= straylight:11434
+
+verify-device:
+	cargo build --workspace --locked
+	BOXMAKER_INFERPROXY=$(CURDIR)/target/debug/inferproxy BOXMAKER_UPSTREAM=$(UPSTREAM) \
+		cargo test -p loopd --test device --locked -- --ignored --test-threads=1
+	@echo "verify-device: ok"