CI: stop rebuilding the framework at every run; a build cache, ccache, and the version out of the compiler flags (#74)

A pull request's firmware step took 358 s: 260 of them rebuilding the
framework that was already in the volume, because the platform decides by
sdkconfig.defaults in the project folder, which is generated and not in git,
so no fresh checkout had it. It is now kept in the volume, inside the
libraries it describes, and copied into the checkout; the platform still
checks its hash against platformio.ini.

The version was a -D on every command line: each commit recompiled
everything, and no cache could help. scripts/version.py now writes one
generated header, read by one file.

PlatformIO's build cache in the volume for pull requests' firmware, ccache
for the host tests (built for coverage, which the build cache can't keep),
the tools in a venv in the volume, and a tag builds its firmware once.
A release still compiles its own sources from nothing.

Measured on fresh copies of the tree: the firmware step 358 s to 27 s (a new
version and one changed file), tests and coverage 49 s to 33 s, a local
rebuild with nothing changed 77 s to 13 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

site/content/dev/build/build-and-test.md

@@ -22,7 +22,7 @@ This runs the host-side unit tests (`test/`, `native` environment), then builds
 
 `scripts/coverage.sh` runs the same tests with coverage counters and writes a line-by-line report to `.pio/coverage/index.html`. The badge above is its figure for `main`: the share of the lines of `lib/` that the host tests run. `lib/` is the logic that compiles on a PC; `lib/SD` (the card's driver) and `src/` (the Apps, the Services, everything that needs the device) have no host tests and aren't in that figure.
 
-The framework is rebuilt with the TLS settings in `platformio.ini` (`custom_sdkconfig`, ADR 0006), so the first build after a fresh checkout takes about 4 minutes; later builds take under a minute.
+The framework is rebuilt with the TLS settings in `platformio.ini` (`custom_sdkconfig`, ADR 0006), so the first build after a fresh checkout takes about 6 minutes; later builds take about 15 seconds when little has changed. The platform knows the framework is already rebuilt by `sdkconfig.defaults` in the project folder, which it writes and git ignores: delete it and the next build rebuilds the framework. CI keeps that file, a build cache and ccache in its volume (docs/milestones/R1.md).
 
 ## CI and releases