proof/: add StarForth_Block_Words.thy (SCR only)

block_words.c is categorically different from every file covered so far in
this sweep: every other word_source file operates on pure per-VM internal
state already in vm_state (data_stack/return_stack/memory/dictionary).
block_words.c sits on top of a real disk-backed I/O subsystem
(block_subsystem.h) plus a per-VM in-memory cache of it
(vm->blk_vm_lbn/blk_vm_cbuf/blk_vm_dirty/blk_vm_next), none of which are
in vm_state.

Only SCR is self-contained (just needs vm->scr_addr, added to vm_state
the same way here/ecw_nesting were for earlier files). The other 11 words
are deferred for three reasons documented in the theory header: the
block-window cache subsystem (a modeling project on the scale of the
deferred TIB input subsystem, not a one-word extension), real disk I/O via
block_subsystem.h, and recursive vm_interpret()/printf() in LOAD/LIST/
THRU/-->.

Noted in passing: blk_vm_evict/blk_vm_flush_all's own comments document a
real raw-pointer-lifetime bug (stale C buffer pointers after block-
subsystem struct-copy eviction) that was already found and fixed by hand
in the C, before this suite ever looked at the file -- not an open issue,
just worth recording as prior art for exactly the class of bug this sweep
exists to catch.

30 theory files verify with zero errors.

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

proof/StarForth_Base.thy

@@ -514,6 +514,11 @@ record vm_state =
      nat (C only ever tests ecw_nesting > 0, never negative values in any
      reachable path) -- see StarForth_Double_Words.thy's 2>R/2R>/2R@. *)
   ecw_nesting   :: nat
+  (* ○ CODE-MUST-MATCH: C: vaddr_t scr_addr (include/vm.h:451, vaddr_t =
+     uint64_t) -- VM address of the SCR variable cell holding the current
+     block number. Modeled as nat, matching `here`'s convention for VM
+     addresses. See StarForth_Block_Words.thy's SCR. *)
+  scr_addr      :: nat
 
   (* ── Physics Loop #1: Execution heat tracking ───────────────────────── *)
   (* ○ CODE-MUST-MATCH: heat_threshold_{25th,50th,75th} in C VM struct.