No-op incremental build omits the Flash:/RAM: percentage lines, so size-parsing tooling sees no report #1277

On fbuild 2.5.15, a build where nothing changed prints no Flash: / RAM: percentage lines — presumably because the linker didn't re-run. Reasonable behaviour on its own, but it makes those lines unreliable as a size source for tooling, since the common case once a debounce settles is a no-op build.

Repro

Build twice in a row with no source changes.

First (clean) build:

   0.00 Memory: 31.50KB Flash, 2.00KB RAM
  35.88 Flash: 16.19KB / 31.50KB (51.4%)
  35.88 RAM:   1.82KB / 2.00KB (90.9%)
build succeeded in 35.9s (flash: 16580 bytes, ram: 1862 bytes)

Second (no-op) build:

   0.00 Memory: 31.50KB Flash, 2.00KB RAM
build succeeded in 0.3s (flash: 16580 bytes, ram: 1862 bytes)

The Flash: / RAM: lines are gone; the summary line and the Memory: line remain.

Impact

Design Studio for FastLED shows a live controller-capacity meter driven by a compile-only build while the user edits a graph. Parsing Flash:/RAM: meant a settled no-op build silently produced no reading at all, so we fall back to reading fbuild's own .fbuild/build/<env>/release/.firmware_size_cache.json — which works, but couples us to an internal file.

Possible resolutions, in preference order

1. Reprint the cached size report on a no-op build. The numbers are known — the summary line already carries them.
2. Document Memory: + build succeeded (...) as the stable pair. Both are printed on every build, and together they give used bytes and the limit, which is everything needed for a percentage:

   Memory: 31.50KB Flash, 2.00KB RAM              <- limits
   build succeeded in 0.3s (flash: 16580 bytes, ram: 1862 bytes)   <- used
   

   If that pairing is intended to be stable, we'd happily parse those instead and drop the cache-file dependency entirely.
3. A --size-report / machine-readable flag, if one is planned — fbuild symbols --json already covers the per-symbol case nicely, but it needs an ELF and is heavier than a headline figure.

Option 2 costs nothing if those lines are already contractual; a one-line note in the docs would be enough for us to switch.