Synty POLYGON Military characters for both factions - #23

Replaces the generated and CC0 characters on the bots with purchased Synty
POLYGON Military models: `Insurgent_Male_01` for Nightcell, `Soldier_Male_01`
for the Directorate. The pack has both by name, which is what neither the
generated set nor the generic CC0 pack could offer — the best that pack had was
an `Adventurer` standing in for an insurgent and a hi-vis `Worker` before it.

~2k triangles each on a 55-bone Unreal-standard skeleton, against 5.9k for the
generated character. Total shipped assets fall to 5.13 MB against a 9 MB guard.

Adds `import_synty.py`, which does two things the plain glTF path cannot:

  - Retargets animation. Synty's SourceFiles carry a bind pose only; the cycles
    live in the engine-locked Unity and Unreal packages. The script maps our
    CC0 clips onto the Unreal skeleton with copy-rotation constraints and bakes
    them. Copying f-curves directly does not work — curves are in each bone's
    rest space and the rigs do not share rest orientations, which produces a
    figure with its arms through its chest.
  - Shares the atlas. Synty puts the whole pack on one 4096 texture; embedding
    it per character cost 2.48 MB each, and binding it once by material name
    costs 357 KB total with characters at ~650 KB.

Two details worth keeping: the baked clips take the source clip names only
after the source is renamed out of the way, because Blender appends `.001` and
the engine looks clips up by exact name; and the renamed sources are deleted
before export or they ship alongside and double the animation count.

Source files are not committed. The pack is 406 MB unpacked and lives outside
the repo; PROVENANCE.md records the purchased licence.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>