[plenarius]

Follow-ups to #109 from layonara/nwn-module#476 and #55.

Root transform.

• The compiler never wrote position or orientation controllers on the model root. The game's compiler writes each when the ASCII declares it (explicit zeros too), and nothing otherwise.
• New engine fixtures zcm_rootpos, zcm_rootzero and zcm_rootnone are the game's compiles of those three cases. The compile now matches all three, and they fail without the fix.
• Layonara's ttr01_z01_01 (root at z −1.75) moved up 1.75 m in a round trip. Of the game's ASCII models, only dag_chest declares a root transform.

Zero-length tangents.

• The game draws a mesh's tangents as given, and on a normal-mapped mesh a zero-length one shows as a white streak. That's Layonara's Center Craft Hall: tin01_l16_06 had 7 to 57 per mesh, from its source file and from values the parser could not read.
• compile keeps them, as the game does, with a warning.
• check's tangent_validation reports them, and drops them with --fix.
• repair --fix-tangents, part of --all, drops a mesh's tangents when any has zero length, so the compiler computes them from the geometry. That's what the game does for a mesh without tangents; on tin01_l16_06 the computed ones have none of zero length.
• Binaries were already safe: the decompiler drops a tangent stream that isn't unit length unless the header asks for tangents.

parity_compile --cli passes --force when the CLI has it. Since #109 the CLI refuses to write a binary whose render lists the mutations damaged, while the library compiles it, and that comparison is about the bytes. The fixture-only CI run doesn't use --cli, which is how #109 missed it.

Testing.

• In game: tin01_l16_06 as it was before haks PR fix(compiler): don't split skin vertices at material boundaries #23 (with the zero tangents), repaired with --fix-tangents and compiled: no white strips in the Center Craft Hall.
• Corpus parity on the game's models passes: decompile (25,597), parse (7,235 plus mutations) and compile (32,832, also through the CLI).
• New tests: the three root fixtures, TestZeroLengthTangents (compile warning, drop, recompute) and TestTangentValidationZeroLength.

Ported to python/: the root rule, the declared keywords and the compile warning. README lists --fix-tangents and --force.
Qt: plenarius/cleanmodels-qt (feat/fix-tangents). Spec: plenarius/nwn-mdl-spec (spec/root-controllers).