fix(rig): curl the fingers, derive the arm angle

Two defects from the 0.2.0 review, both fixed and both measured.

- Fingers splayed because the bind is an A-pose hand. All three joints of all five
  fingers per hand are curled in the standing base (0.28/0.34/0.22 rad), with the
  curl axis probed on the rig first. The front shot now shows relaxed hands.
- The arms-down rotation is derived on every build instead of hard-coded: the
  builder sweeps candidate angles and returns the one whose bone direction points at
  the target (0.60 rad, direction dot 1.000 this build). The old 0.55 was a
  hand-swept constant.
- Base bones are keyed in both clips, so a clip that only animates the limbs cannot
  drop the standing offset.

Regression, recorded rather than hidden: the stride shot shows a standing pose again
after this change. The probe had shown the walk clip playing (speed=28
clip=GigLine_Walk) before it, and pinning the clip to a quarter cycle did not restore
the stride, so the cause needs a fresh look. The shot is retired rather than shipped
broken, and the known-issues table now points at it.

Tools/ue_character_anim_import.py

@@ -19,7 +19,7 @@ import unreal
 
 SRC = "/tmp/gigline/character-anim.fbx"
 DEST = "/Game/Test/Character"
-PREFIX = "CharacterV2"
+PREFIX = "CharacterV3"
 RESULT = "/tmp/gigline/character-anim-import.txt"
 lines = []