Marker remediation: SAFETY comments, #[allow] rationales, INVARIANT markers

[shinaoka]

Part of #199 (Phase 3, unsafe hygiene). All file:line references are pinned to origin/main at 9da9b9f. Follow-through of the marker rules once adopted.

Measured today: // SAFETY: on 29 of 418 unsafe blocks (strided-view 0 of 23, strided-perm 0 of 27, kernel 24 of 269); // INVARIANT: zero; 33 of 35 #[allow] without adjacent rationale.

Priority sites where the proof is real but remote and unwritten (from the audit): erased.rs:2036, :2067, :2105, :2347, :2603 (new_unchecked from erased descriptors; proof lives in validate_bounds at descriptor construction), static_indexing_plan.rs:161, :186, map_view.rs:1847 (broadcast strides), the uninit blocks listed in the audit at static_indexing_plan.rs:462, :613, :668, copy_plan.rs:194 (execute_uninit_then, the RMW receipt producer, has no rustdoc at all), and the two SAFETY comments that overstate what was checked (gather_plan.rs:736, :1025).

• SAFETY comment per unsafe block naming the validation site that proves it.
• Rationale line per #[allow].
• INVARIANT markers on the intentional perf patterns the audit flagged as future false-positive bait (rank-bounded loops, unreachable! guarded by supports_*, extent-spanning receipts).