FABRIC-2.md: arch_mmu_init() is a deliberate TODO, not dead code

Investigated 2026-08-13: confirmed unreachable by repo-wide grep, but
each of the three stubs is a real doc-commented placeholder for a future
per-arch MMU milestone, not leftover cruft. Captain Bob's call: leave in
place, keep tracked as a TODO rather than delete or wire in a no-op call
site. No code changed.

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

FABRIC-2.md

@@ -146,8 +146,17 @@ 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()` is declared and defined on all three architectures but never called
-  anywhere — dead code. Found during item 4.3.5a.
+- [ ] **`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
+  milestones" / "MMU bring-up deferred" / "Sv39/Sv48 bring-up deferred"), and each comment
+  describes it as satisfying "the common `arch_mmu_init()` call site shared across all three
+  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.
 - [ ] `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)