Implements PyramidHART, the ℓ₁ cooperation statistic partitioner from Peace (2026).
Uses A = |S| − ‖z‖₁ as its partitioning target — a polyhedral zero boundary whose
sign-consistent octants form a double pyramid in d=3, vs the smooth T=0 cone of HART.

New module src/hvrt/_geometry.py:
  compute_S, compute_Q, compute_T, compute_A, minority_sign_total, geometry_stats
  All four cooperative geometry statistics and their cross-relationships; pure functions,
  model-independent, exported from hvrt top-level.

New class PyramidHART(_HARTBase):
  Inherits MAD normalisation + absolute_error tree criterion from _HARTBase.
  Overrides _compute_x_component → compute_A (degree-1 homogeneous, outlier-robust).
  Adds geometry_stats() with per-partition A/T/Q breakdown and Proposition 1 metrics.

Proposition 1 properties verified in 44 new tests (tests/test_pyramid_hart.py):
  A ∈ [−r√d, 0], degree-1 homogeneity, minority-sign = −A/2, outlier cancellation,
  corr_TQ ≈ 0 for isotropic data (Theorem 1), corr_AQ nonzero (Prop 1 trade-off).

notebooks/pyramid_hart_geometry.py: standalone residual-pyramid visualisation (PDF).
Version bump 2.10.0 → 2.11.0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>