Add FENCE word (SDK v1.9.0 scoping); fix severe pre-existing FORGET use-after-free

FENCE ( -- ) exposes the dict_fence_latest/dict_fence_here state FORGET
already honored internally, letting callers (e.g. a future SDK capsule)
raise the boundary after loading their own content -- no new VM fields,
no policy logic beyond exposing existing state.

Writing a direct test for it surfaced a real, severe, pre-existing bug in
FORGET's relink logic, unrelated to FENCE itself and reproducible with the
original boot-time fence alone:

- Forgetting the single newest word incorrectly destroyed every other word
  back to the fence too, not just the target.
- Forgetting an older word (correctly cascading to remove newer words too,
  per FORTH-79 semantics) crashed with SIGSEGV.

Root cause: the relink code's target_prev pointer was, by construction,
always inside the range the preceding loop had just freed whenever target
wasn't vm->latest -- so writing through it was a use-after-free every time
that branch executed. Fixed by removing the target_prev tracking and both
branches entirely; vm->latest unconditionally becomes target_next (target's
own captured, still-valid link) after the free loop, correct in every case.

Added a FENCE test suite to dictionary_manipulation_words_test.c (Module 14)
including the exact regression case (forgetting the newest word must not
disturb an older one). Verified zero warnings and identical POST/dict_hash
results across all three kernel architectures.

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

capsules/BLOCK_MAP.md

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