proof/: add StarForth_System_Words.thy (system_words.c coverage)

10 of 16 registered words modeled (COLD/WARM/BYE/WORDS/VLIST/PAGE/NOP/
QUIT/ABORT/EXECUTE guard-shape), plus the internal (ABORT") runtime
helper. Deferred: ( and \ (TIB dependency), SAVE-SYSTEM (real file I/O),
79-STANDARD (reads a non-per-VM global), ABORT" compile-time half (TIB +
codegen), SEE (TIB + raw threaded-code pointer walk).

Two findings. (1) system_running/forth_79_standard are file-scope C
statics doing per-VM-shaped work -- the 5th and 6th occurrence of this
bug pattern in the sweep (previously: control_words.c's cf_stack,
dictionary_manipulation_words.c's state_variable, string_words.c's
word_scratch_addr). (2) EXECUTE casts a popped FORTH cell straight to a
DictEntry host pointer and calls through it (entry->func(vm)), gated
only by a null check -- same hazard class as format_words.c's ?/DUMP but
far more consequential since EXECUTE is a core, ubiquitous primitive
rather than a diagnostic word. Flagged as the highest-severity finding
this sweep has produced.

proof/ROOT

@@ -18,6 +18,7 @@ session "StarForth" = "HOL-Library" +
     StarForth_IO_Words
     StarForth_Editor_Words
     StarForth_Format_Words
+    StarForth_System_Words
     StarForth_Mutex
     StarForth_Transition
     StarForth_Loop1_Heat