build: sdr-host Corepack + docs + dockerignore audit (3/3)

[Coriou]

Summary

Third of three stacked PRs. Stacked on #2 — base is build/devx-overhaul-pr2; cascades to main automatically as #1 and #2 merge.

Polish phase: sdr-host alignment, docs consolidation, and a substantive .dockerignore fix that turned out to be more than the spec anticipated.

• Phase G (chore(sdr-host): switch pnpm install to Corepack pattern) — single-line edit to packages/sdr-host/Dockerfile: RUN npm install -g pnpm@10 → RUN corepack enable && corepack prepare pnpm@10.28.0 --activate. Matches the main Dockerfile pattern and pins explicitly to package.json's packageManager field. Verified via docker buildx build -f packages/sdr-host/Dockerfile --target node-build --platform linux/amd64 --load . — Corepack steps succeed, pnpm install --frozen-lockfile resolves 102 packages, all 3 workspace tsc builds pass. Arm64-via-QEMU is arch-agnostic for this pattern; sdr-host's per-host CI / make sdr-host-build exercises full arm64.
• Phase H (docs: consolidate Docker documentation around compose profiles) — docs/DOCKER-SETUP.md rewritten as the canonical reference: pnpm dev as headline, compose profiles, GHCR cache, demod-test environment, R7 disclosure (first-run SourceConnectionError noise is expected — s6 hard dep removed; SourceManager's exponential backoff is the authoritative source-availability mechanism). docker/README.md deleted (294 lines that duplicated DOCKER-SETUP.md content, mostly stale). CLAUDE.md's "Day-to-day dev (Docker)" section condensed from a 14-line block + code fence to one paragraph that names the new canonical targets.
• Phase I (chore: audit .dockerignore effectiveness; fix stale doc reference) — substantive .dockerignore fix: top-level globs (dist, tests, .turbo, .pnpm-store, coverage, .tsc-cache) were silently failing to match workspace mirrors (cli/dist, packages/sdr-host/tests, etc.). Added **/... siblings for all of them. Build context size dropped from 1.70 MB / 348 files to 15.56 KB. This was a genuine bug; the spec's prediction that Phase I might be a no-op didn't hold. Also fixed one stale doc reference: docs/DECODER-GUIDE.md:555 make dev-logs → make dev-stack-logs (Phase E renamed the target).

Final-gate Non-negotiables: 13 PASS / 2 DEFERRED

Spec lists 15 (tasks.md lines 199–218). All 13 testable items pass; the 2 deferred match this execution's no-push directive until the PRs landed.

#	Spec	Result	Evidence
1	pnpm dev no Docker	PASS	Background-launched, app booted in ~16 ms post-restart, zero docker subprocesses
2	warm-cache no-op	PASS (amd64) / DEFERRED (multi-arch)	Single-arch: 8.965 s → 7.163 s, 124 CACHED stages on 2nd run. Multi-arch needs CI
3	TS edit invalidates only node-build + finals	PASS	All 12 decoder *-build stages CACHED on touch src/index.ts rebuild
4	one-decoder edit invalidates only that decoder	PASS	Bumping READSB_REF short-circuits at readsb-build with not our ref (proves cache key isolation)
5	no plain docker build	PASS	grep -rn '\bdocker build\b' docker/ Makefile compose.yaml | grep -v buildx empty
6	one compose file	PASS	Only ./compose.yaml at root
7	final-core zero sdrpp residue	PASS	find /etc/s6-overlay -iname '*sdrpp*' empty, no sdrpp in /usr/local/bin or /usr/bin
8	wavekit-api deps minimal	PASS	exactly wavekit-init in dependencies.d/
9	final has exactly one sdrpp-server dir	PASS	one match for find /etc/s6-overlay -name sdrpp-server -type d
10	CI runs both jobs on PR	DEFERRED	This PR's CI run IS the validation
11	no emoji in newly-introduced output	PASS	git diff for Makefile/docker/.github shows no emoji added
12	4 deleted compose files gone	PASS	all 4 listed in git diff --diff-filter=D
13	demod-test Dockerfile + compose gone	PASS	both listed in git diff --diff-filter=D
14	packages/sdr-host/docker-compose.yml unchanged	PASS	0-line diff vs spec-commit baseline
15	sdr-host Corepack	PASS	grep -c corepack packages/sdr-host/Dockerfile = 1, npm install -g pnpm count = 0

Test plan

• CI green for build: s6 tree partition + Dockerfile refactor (1/3) #1, build: collapse compose, adopt bake, rewrite Makefile + CI (2/3) #2, build: sdr-host Corepack + docs + dockerignore audit (3/3) #3 in order (stacked merges)
• Reviewer confirms docs/DOCKER-SETUP.md covers the four headline topics (pnpm dev, dev-stack, GHCR cache, demod-test) + the R7 disclosure
• Reviewer confirms .dockerignore build-context reduction (run docker buildx build --target base-build --progress=plain . and observe "transferring context: ~15kB" for a clean tree)
• Reviewer confirms packages/sdr-host/Dockerfile has corepack enable && corepack prepare pnpm@10.28.0 --activate with no npm install -g pnpm remaining

Post-merge

• D.8 (initial GHCR cache seed): happens automatically when this stack merges to main and the CI's main-branch path runs with CACHE_FROM_ONLY=false + push: true + cache-to mode=max for every cache ref.
• The first CI run on main will take 25–40 min (cold). All subsequent runs (PR or main) will hit the seeded cache and complete in 2–5 min for typical changes.