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

lib/version/src/version.cpp

@@ -0,0 +1,15 @@
+#include "version.h"
+
+// Written by scripts/version.py before each build; not in git.
+#if __has_include("version_generated.h")
+#include "version_generated.h"
+#endif
+#ifndef RORO_VERSION
+#define RORO_VERSION "unknown"
+#endif
+
+namespace roro {
+
+const char* versionString() { return RORO_VERSION; }
+
+}  // namespace roro

lib/version/src/version.h

@@ -1,14 +1,12 @@
 #pragma once
 
-#ifndef RORO_VERSION
-#define RORO_VERSION "unknown"
-#endif
-
 namespace roro {
 
 constexpr const char* kProductName = "roro9stack";
 
-// "roro9stack v0.1.0" — used on the boot screen and in About.
-inline const char* versionString() { return RORO_VERSION; }
+// "v0.1.0", from `git describe` (scripts/version.py): used on the boot screen and in About.
+// A function in one file, not a macro on every compiler command line: a new commit then recompiles
+// that one file, and everything else comes from the build cache (issue #74).
+const char* versionString();
 
 }  // namespace roro