The previous commit let users cycle empty matrix cells in the modal but
the engine only iterated `payload.matrix` — overrides for keys absent
from the library were silently ignored. Cycling produced dead UI: state
saved, engine ignored, runtime identical.
Engine extension: after the library-merge pass, do a second pass over
`cell_mode_overrides` ∪ `cell_factor_overrides` minus `covered_keys`.
Each leftover key becomes a new matrix entry with implicit baseline
mode "allow" + factor 1.0; whichever side(s) the user set populate the
rest. Disabled set still wins — entries in `disabled_matrix_cells` skip
both passes.
Added `decode_key` helper alongside `encode_key` for the second-pass
src/tgt extraction. Round-trip + malformed-input rejection covered by
new key tests. Malformed override keys in the resolve path are skipped
silently rather than crashing the run (defensive — instance JSON could
have rotted across schema migrations).
Updated the existing "unknown key silently ignored" test to assert the
new behavior (entry created from override). Added focused tests for:
factor-only override defaults to allow mode, disabled set still skips
empty-cell overrides, malformed key in override map doesn't crash.
DumiFlex