box3d: unchecked-hulls opt-in feature + algebraic-float-ops evaluation

unchecked-hulls (off by default): elides bounds checks on hull-topology
indexing in the three SAT hot loops via a cfg'd accessor. Safety contract
= hull connectivity invariants validated at construction (hull.rs
is_valid_hull_impl + create_hull asserts), immutable behind Arc; debug
builds always assert, so every test run exercises the contract. Measured
(paired, retrained PGO): junkyard -3.6%, washer -2%, nothing elsewhere —
documented honestly that the checks were NOT most of the hull residue.
Tests 179 green with and without the feature; hash 0x61E35C31 both.

README: evaluation of the newly-stabilized algebraic float ops idea —
incompatible with the determinism contract as a default (compiler-
version/ISA-dependent results break cross-arch equality and cross-build
replay), modest expected upside since hot paths are already hand-
contracted; possible future opt-in, not planned.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>