feat(build): report flash and RAM footprint after a build

[srpatcha]

The MLP developer walk in the platform design document ends with a build that says how much of the board it used:

Flash: 384 KB
RAM:    72 KB
Ready to flash.

Nothing produced those numbers. A developer had to run size themselves and remember which columns to add — that is not being told, it is being left to find out.

$ ebuild build
[ok] Build completed successfully.

  Flash:     1.9 KB  of    1.00 MB  (0.2%)
  RAM  :   300.6 KB  of   192.0 KB  (156.6%)
[warn] RAM usage 300.6 KB exceeds the board's 192.0 KB -- this image will not fit.

Accounting

Matches scripts/measure_footprint.py in the eos repo, so the two tools cannot disagree about what a number means:

flash = text + data
ram   = data + bss

data is charged to both — it is stored in flash and copied to RAM at startup. Reading size's dec column instead understates RAM.

Where capacity comes from

The project's own board.yaml memory.flash_size / ram_size when it ships one — the convention the descriptions under hardware/board/ already use — otherwise a table of the reference part per board family.

Boards that boot from removable storage, and Linux-class parts with no fixed budget, are deliberately absent. A percentage against a guessed ceiling reads as authoritative; those report absolute sizes only.

Decisions worth reviewing

• Over budget is a warning, not a failure. The image linked. It will not fit. That is better heard now than from a board that will not boot.
• A cross build never falls back to the host size. Host size on an ARM ELF reports numbers for a different target and nothing in the output would say so. No tool → no report, rather than a wrong one.
• A footprint that cannot be measured never fails the build. It is a missing convenience, not a defect in the user's code.

Also fixes .gitignore — found while committing this

Line 8 was a bare build/ from the Python-packaging block. It also matched ebuild/build/, the source package holding dispatch.py, ninja_backend.py and toolchain.py:

$ git check-ignore -v ebuild/build/footprint.py
.gitignore:8:build/	ebuild/build/footprint.py

Those four files predate the rule and stayed tracked, but every new module added there was silently ignored — including this one. Anchored to /build/, which keeps the setuptools artifact directory ignored and stops the pattern reaching nested source.

Verification

pytest: 235 passed, up from 202; 32 are new here. Built and measured three real projects — within budget, over budget, and a board with no known capacity — plus a gcc-compiled binary with a known 4 KB bss array to confirm the section accounting.

Branches off the master repair in #70, since master cannot currently run its own test suite.

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