refactor(theme-studio): defer generate.py's page build behind a lazy _build()

Importing generate.py ran the full assembly at module load -- DEFAULTS, the
package inventory, palette/syntax/uimap construction, and the HTML fill -- so
face_coverage.py, which imports it only for UI_FACES, paid the whole cost. Move
that derivation into a cached _build(); expose the built attributes (HTML, MAP,
APPS, ...) lazily via PEP 562 __getattr__. UI_FACES/CATS/COLS and the file bodies
stay cheap module constants, the file write stays __main__-guarded, and the page
is byte-identical (check-generated current; test_generate green via __getattr__).

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