Support single-foot ground-lock and warn on unsupported ground-lock effectors #60

Summary

A user trying to author a standing crossover turn cannot pivot on one planted foot because ground-lock currently supports the grouped values feet / hands, but not the per-side effectors foot_right / foot_left.

The production playground silently accepts the unsupported values and reports “parses clean, no warnings”, so the movement looks like turn is broken even though the problem is the ignored/unsupported ground lock.

Verified on https://posecode.org/play on 2026-07-15:

• a control movement using turn: 180 with ground-lock: feet parses and the figure rotates;
• changing the lock to ground-lock: foot_right still reports “parses clean, no warnings”;
• the vocabulary/spec currently documents ground-lock groups, while per-side foot effectors are already supported for reach: / pin:.

Reproduction

posecode exercise "Standing Crossover Turn"

rig humanoid

pose start = standing

step "Left crossover step" 0.55s ease-in-out:
  hip_left: adduct 28
  knee_left: flex 42
  ankle_left: dorsiflex 9
  knee_right: flex 6
  turn: 70
  ground-lock: foot_right
  cue "Lift the left foot and cross it over the right while balancing on the right leg"

step "Pivot and settle" 0.65s ease-in-out:
  hip_left: adduct 0
  knee_left: flex 0
  ankle_left: dorsiflex 0
  knee_right: flex 0
  turn: 180
  ground-lock: foot_right
  cue "Pivot on the right foot, uncross the left foot, and settle with both feet under the hips"

step "Right crossover step" 0.55s ease-in-out:
  hip_right: adduct 28
  knee_right: flex 42
  ankle_right: dorsiflex 9
  knee_left: flex 6
  turn: 250
  ground-lock: foot_left
  cue "Lift the right foot and cross it over the left while balancing on the left leg"

step "Pivot and finish" 0.65s ease-in-out:
  hip_right: adduct 0
  knee_right: flex 0
  ankle_right: dorsiflex 0
  knee_left: flex 0
  turn: 360
  ground-lock: foot_left
  cue "Pivot on the left foot, uncross the right foot, and settle evenly on both feet"

repeat 1

Actual behavior

• The editor reports “parses clean, no warnings.”
• foot_right and foot_left do not provide single-foot grounding.
• The authored crossover turn therefore cannot pivot around the intended planted foot.

Expected behavior

1. Support per-side ground locks:
   • ground-lock: foot_left
   • ground-lock: foot_right
   • ideally the equivalent hand aliases if the ground-lock model is generalized consistently.
2. Until/if an effector is supported, emit a line-anchored parser/editor diagnostic instead of silently accepting it.
3. Keep ground-lock: feet and ground-lock: hands behavior backward compatible.
4. Update completions and the spec to list the accepted ground-lock values.

Acceptance criteria

• A phase locked to foot_right keeps only the right foot planted while the left foot can cross or step.
• A phase locked to foot_left behaves symmetrically.
• Unknown or unsupported ground-lock values produce a visible, line-specific diagnostic.
• Parser/render tests cover grouped values, per-side values, and an invalid value.
• The reproduction above can visibly pivot around the alternating planted foot without reporting false-clean validation.