Phase 8 v3: refuse a same-VM, cross-device raw block copy from within StarshipOS

Two research passes confirmed the identity record itself (seed/pubkey/
cert) is already unreachable from any FORTH primitive -- only C-level
read_devblock/write_devblock touch it. But a device's ordinary user block
content CAN be copied between two attached devices today, using only
stock, unpinned words: <src> BLOCK <dst> BUFFER 1024 MOVE UPDATE
SAVE-BUFFERS, or the dedicated RELOCATE-BLOCK word (whose own doc comment
already admits "performs no policy validation of its own"). Checked
whether the existing per-block owner_fp/BLK-ACL-ALLOW@ metadata already
solves this -- it doesn't: owner_fp encodes who (a VM identity pubkey),
never where (physical device), and blk_get_buffer()/blk_update() never
consult acl_allow/acl_ttl at all -- those fields are completely inert.

Small, targeted fix, no rearchitecture:

- blk_subsys_relocate_block() (block_subsystem.c): same-device check.
  Its own documented purpose is wear-leveling (relocate on the SAME
  device) -- never stated as cross-device, and nothing enforced that
  until now.
- New public blk_lbn_device_handle() (block_subsystem.c/.h): the missing
  LBN-to-device direction (blk_get_device_range() already goes the other
  way). Opaque, stable, == comparable.
- MOVE (memory_words.c) and CMOVE/CMOVE> (string_words.c): refuse when
  both addresses are block-window addresses backed by two different
  devices -- the exact shape of the composed attack. A copy where either
  end is ordinary VM memory (the overwhelming common case: staging text
  from PAD, editing a block in place) is untouched.
- blk_vm_check_epoch()/blk_vm_slot_for_addr() exposed (block_words.h) so
  the two new call sites share the same window-slot invalidation contract
  rather than a second, divergent copy of it.

Verified live on all three architectures, not just boot-clean: same-
device MOVE/RELOCATE-BLOCK still succeed exactly as before; cross-device
MOVE/CMOVE/RELOCATE-BLOCK all refused. Zero UNKNOWN WORD, identical
dict_hash across all three (this change adds no FORTH-visible word, only
internal refusal conditions, as predicted).

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

include/block_subsystem.h

@@ -385,6 +385,18 @@ int blk_subsys_relocate_block(uint32_t home_lbn, uint32_t target_lbn);
  */
 int blk_get_device_range(struct blkio_dev *dev, uint32_t *out_start_lbn, uint32_t *out_count);
 
+/* blk_lbn_device_handle - the missing direction from blk_get_device_range()
+ * above (that goes device -> LBN range; this goes LBN -> device). Returns
+ * an opaque, stable handle -- directly comparable with == to test "do
+ * these two LBNs belong to the same physical device" -- or NULL if lbn
+ * doesn't resolve to any attached device. Never dereference the returned
+ * pointer; it exists only for equality comparison (Phase 8 v3, 2026-09-23:
+ * added so MOVE/CMOVE/CMOVE> can refuse a same-VM, cross-device raw block
+ * copy through the block-window mechanism -- see those words' own callers
+ * in memory_words.c/string_words.c).
+ */
+const void *blk_lbn_device_handle(uint32_t lbn);
+
 /* blk_get_device_free_blocks - free vs. total 1 KiB FORTH-block count for
  * a specific already-attached device (same slot lookup as
  * blk_get_device_range()). RAM-backed slots (no on-disk vol_meta) report