Add a forefire-validate landscape checker

[HugoFara]

Closes#169.

A forefire-validate landscape.nc fuels.csv command that checks a landscape
against its fuel table before a run — the cheapest fix for the most confusing
failure mode, where an unmapped fuel index makes the simulation fail or produce
wrong results with no message.

It reports any fuel index present in the raster but missing from the table,
flags a fuel/elevation shape mismatch, and warns when elevation or wind is
absent (exit non-zero on any error).

Design:

• Decision logic is pure and I/O-free (build_report); a thin netCDF4
  adapter (read_landscape) asks for netCDF4 only when a file must be read,
  so the wheel gains no hard dependency.
• Exposed as the forefire-validate console entry point.
• test_validate.py loads the checker from source and covers the pure logic
  without the compiled extension; the real-.nc case is skipped when netCDF4
  is absent. Wired into run.bash; documented in TESTING.md and the Python
  README.

Note: the entry point is new pyproject.toml metadata, so it appears after a
reinstall/rebuild — worth a CHANGELOG line at the next release.

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