[plenarius]

Fixes the cause of layonara/nwn-module#476: walls lost their lower parts after cleanmodels repair (haks PR #23).

Cause. Some binaries draw meshes of one texture as one. The receiving mesh's render index list (MDL-BIN-049) holds its own faces' triangles plus copies of the others' (the donors). Each donor keeps its faces, but has render 0 and an empty index list. The game draws index lists, so the model looks whole. The decompiler read only faces, so the copies were lost and the donors were written invisible.

Fix.

• The decompiler reads each mesh's index list (pkg/mdl/binary_merged.go). It writes every donor whose triangles are all among a receiver's copies with render 1, which draws the same triangles as a mesh of its own.
• Triangles are matched by texture coordinates and side lengths (within 1 mm), so placement doesn't matter; untextured meshes match by shape alone.
• Each receiver gets a note, and a donor that an animation moves gets one saying it now moves. That happens in 6 base-game dag_* placeables.
• Triangles no donor accounts for get a warning, and repair, compile and check --fix refuse to write the model unless --force is given. decompile keeps its own --force. A test now checks that every subcommand's flags register.
• Ported to python/.

Scale. 1,015 base-game binaries (tno01, tts02, ttf02 and tcn01 tiles first) and 342 in Layonara's haks hold merged meshes. All of them are put back fully, with no triangles lost.

Round trip (decompile, then compile) compared with the original, by the triangles drawn in world space:

	v4.12.0	this PR
1,015 game binaries	47% of drawn triangles lost	every triangle drawn, except degenerate ones
342 haks binaries	34% lost	the same

Testing.

• In game (Layonara shop PH_000195, the three tiles from #476): repaired with this build the way PR fix(compiler): don't split skin vertices at material boundaries #23 did it, the walls are whole.
• Parity (parity_test.py on the game's 25,597 binaries) passes. Go and Python give the same warnings and notes on all of them, and the same ASCII, warnings and notes on the 342 haks models.
• The conformance suite's results (46 compile, 46 decompile, 14 stock) are unchanged.
• New tests cover matching, untextured donors, lost triangles, animated donors, the refusal, and flag registration.

Spec: plenarius/nwn-mdl-spec MDL-BIN-053 (separate PR).