feat(warp-core): footprint enforcement guard (Phase 6B)

[flyingrobots]

Summary

• FootprintGuard: Runtime enforcement that validates all WarpOps against declared read/write sets during BOAW parallel execution. Rules must declare their footprints via compute_footprint, and the guard catches violations at the reserve phase before execution begins.
• Complete footprint declarations: All existing rules (motion, toast, route_push, set_theme, toggle_nav, drop_ball, ball_physics, put_kv, inbox) now have correct and complete footprint functions.
• Slice theorem proof tests: New test suite proving deterministic parallel execution — same state roots regardless of worker count or execution order.

Key Design Decisions

• Guard is active in debug builds and behind footprint_enforce_release feature flag
• panic_any with typed FootprintViolation payloads enables structured error reporting
• Conservative over-declaration of reads is safe (only under-declaration triggers violations)
• The ExecItem::kind field is now private to enforce construction through ::new()

Test plan

• cargo clippy --all-targets — clean (no errors)
• cargo test --workspace — all 595 tests pass
• echo-dind-harness coverage + permutation invariance tests pass with enforcement active
• Pre-push benchmark suite passes (fmt, clippy, tests, rustdoc, patterns, determinism)

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