Phase 1 / Forge / Forge 3D narrowphase fast paths

[guysenpai]

M1.1.4 — the four analytic per-pair narrowphase fast paths (fifth M1.1 sub-milestone), dispatched from collideOrdered before GJK: sphere/sphere, sphere/box, capsule/capsule, box/box (radius-0 box only; a rounded box falls through to the generic path).

Architecture

Each kernel computes only a ContactSeed { normal (A→B), core closest points, base_penetration } fed to the unchanged generateManifold — so the feature_id producers, supporting-face clipping, ≤4 reduction, per-point penetration, order-independence and frame-stability are all inherited by construction. The fast path replaces only the GJK descent + EPA polytope expansion.

• fastSeed — 3-state dispatcher (not_handled / separated / contact); separated mirrors gjk.zig (conv_k=16, coord_scale=|Δpos|+coreExtent).
• sphere/box deep = closed-form least-penetration face (point/box P1d fix, validated >200:1).
• box/box = 15-axis SAT (order-independent; separation tested on every non-zero raw edge×edge axis; edge-edge witness via full-degenerate Ericson closestSegSeg) — fixes box/box P1d and bypasses the M1.1.3 deep-rotated EPA frame-dependence for box/box.
• capsule/capsule = closestSegSeg → 3 regimes via generateManifold (crossed via the E1 generator fix; collinear/parallel radial normal, never axial).
• E1 generator fix (pre-existing M1.1.3 defect): segment × segment non-parallel → single witness.

Threshold-class discipline (hardened across a 5-round external review, all RED-first in-milestone)

• Normalization guards fire only at true zero (floatMin, never a geometric scale).
• Point-set dedup is relative to the candidates' per-axis EXTENT (max−min) — translation + scale + anisotropy invariant.
• Separation margins are k·floatEps·coordScale.

Defects closed RED-first: degenerate-segment closest points, SAT separating-axis completeness, collinear-capsule normal, and the length-threshold class (fast_paths + the shared manifold.zig generator).

Validation

Differential vs collideOrderedGeneric (gated on oracle self-consistency), closed-form P1d / extreme-aspect, oracle-free deep+rotated box/box (order-independence + inline-SAT MTV + frame-invariance), feature_id producer×pair×order matrix, and a scale×translation×anisotropy×decentre×order invariance matrix. Full suite green at f32 and -Dphysics_f64=true, debug + ReleaseSafe. Bench (ReleaseFast, dispatched/generic): 0.55 ss / 0.27 sb / 0.27 bb / 0.12 cc. No gjk.zig/epa.zig touched.

Tracked (out of scope)

• Generic EPA deep-rotated frame-dependence (M1.1.3 defect) → hotfix m1.1.3-hf-epa-frame-dependence; residual exposure: capsule/box + sphere/capsule stay on the generic path.
• Contact-point position accuracy far from body A's centre is f32-limited (a generateManifold precision characteristic).

🤖 Generated with Claude Code