FABRIC-2.md: check off arch_mmu_init() item -- ruling recorded

No code change; closed as investigated and ruled not-a-bug (deliberate
future-milestone scaffolding) rather than left open as a loose TODO.

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

FABRIC-2.md

@@ -146,7 +146,7 @@ and recorded.
   > `capsule_vm_kill()` itself. Three-arch acceptance boot, all clean to `ok>`:
   > `logs/20260813-083429/amd64`, `logs/20260813-083551/aarch64`,
   > `logs/20260813-083738/riscv64`. Committed `db66406`.
-- [ ] **`arch_mmu_init()` — TODO, not accidental dead code.** Declared (`arch.h:73`) and
+- [x] **`arch_mmu_init()` — TODO, not accidental dead code.** Declared (`arch.h:73`) and
   defined on all three architectures (`amd64/arch.c:246`, `aarch64/arch.c:169`,
   `riscv64/arch.c:179`) but never called anywhere — confirmed by repo-wide grep, 2026-08-13.
   Each definition is a real doc-commented no-op stub ("paging will be wired up in later
@@ -155,8 +155,8 @@ and recorded.
   ISAs" — a call site that does not exist anywhere in the tree today. `vmm.c` (M3) handles
   page-table management independently and does not need this hook to function. Captain Bob's
   call, 2026-08-13: leave the three stubs in place as scaffolding for a future per-arch MMU
-  milestone rather than deleting or wiring in a no-op call site now — this item stays open as
-  a TODO marker, not a bug to fix. Found during item 4.3.5a.
+  milestone rather than deleting or wiring in a no-op call site now — ruling recorded, closed
+  as investigated/not-a-bug rather than left open. Found during item 4.3.5a.
 - [ ] `include/block_subsystem.h:168`'s `encoding` field (ASCII/UTF-8/binary) is dead
   metadata — nothing reads or writes it. A ruling on wiring it up vs. removing it is still
   open. (FABRIC.md §27.6)