box3d: check in PGO profile, apply by default via workspace .cargo/config.toml

cargo build --release on anything in the workspace now gets the PGO'd
box3d automatically (-Cprofile-use=libs/box3d/box3d.profdata; verified:
default build runs at the explicit-PGO binary's speed). The profile is
target-independent — x86_64 cross-build with the ARM-trained profile
compiles clean — and degrades gracefully when stale (unmatched functions
keep normal heuristics). Retrain with libs/box3d/pgo.sh.

Fresh four-way interleaved matrix in the README: default box3d is now
faster than or equal to rapier-simd on all three scenes (1118 vs 1451,
1510 vs 1690, 912 vs 914 ms) and faster than non-PGO C on two of three
(C keeps joint_grid 816 vs 912). 179/185/179 tests green with the
config active.

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