FABRIC-2.md: close block_subsystem encoding-field item, wider scope found

Investigated 2026-08-13: not just the encoding field -- blk_get_meta()/
blk_set_meta() and the whole blk_meta_t on-disk layout have zero callers.
Captain Bob's ruling: flag and leave as-is, expected to be consumed once
Artemis's design (content-typed/owned/ACL'd blocks) is completed. No
code changed.

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

FABRIC-2.md

@@ -157,9 +157,20 @@ and recorded.
   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 — 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)
+- [x] **`include/block_subsystem.h:168`'s `encoding` field (ASCII/UTF-8/binary) is dead
+  metadata — nothing reads or writes it.** (FABRIC.md §27.6) Investigated 2026-08-13, scope
+  turned out wider than the field itself: `blk_get_meta()`/`blk_set_meta()`
+  (`src/block_subsystem.c:881,903`) are the only public API for the whole `blk_meta_t`
+  struct and have **zero callers anywhere in the tree** — not just `encoding` but
+  `content_type`, `content_length`, `owner_id`, `permissions`, `acl_block`, `signature`,
+  `entropy`, `hash`, the chain fields, and `app_data[15]` are all unused. The struct is a
+  real fixed on-disk byte layout (packed 341 bytes/block into `META_REGION_OFFSET`,
+  `src/block_subsystem.c:69-71`), serialized via `meta_to_slice()`/`meta_from_slice()`
+  during cache writeback/load — so it isn't free-standing dead code to delete, removing a
+  field would shift every subsequent field's on-disk offset. **Ruling (Captain Bob,
+  2026-08-13): flag and leave as-is** — this metadata subsystem is scaffolding expected to
+  get consumed once Artemis's design (content-typed/owned/ACL'd blocks) is completed, not a
+  bug to fix now.
 - [ ] `tools/README.md` documents a `fbtest.c` example that does not actually exist in
   `tools/` — stale-doc discrepancy.
 - [ ] `hotwords_cache_promote()` has a NULL-write bug on a full cache. Unreachable under