starkernel: item 4.1 -- hot words onto the Stadium, density-ranked eviction

Punch list §25 item 4.1 complete.
Replaces the round-robin hotwords cache with Stadium density-ranked
admission/eviction on the kernel side, via the §17.7 reservoir mechanism and a
kernel-side word_id -> cell_index map (no DictEntry change, dict_hash
untouched). Adds stadium_birth_hera() to close the cell-0 panic hazard,
STADIUM_WORD_HEAT_QUANTUM/STADIUM_WORD_COOL_RATE_Q48 Kconfig knobs (flagged
untuned), and a stadium_word_forget() FORGET coherence hook to close a
recycled-word_id aliasing gap.

Verified: all five hotwords_cache_* call sites in dictionary_management.c
bypassed under __STARKERNEL__; word dispatch feeds the Stadium at all three
vm_core.c physics_execution_heat_increment() sites; hosted make unaffected;
all three architectures booted to ok> with matching dict_hash
(0x3d4e1daf289da94f) and matching conservation stats (promotions=354
evictions=0, resident_sum=65536 reservoir=0 sum=65536).

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

Kconfig.kernel

@@ -93,6 +93,36 @@ config STADIUM_MEMORY_PERCENT
 	  against a QEMU -m 1024 test config while leaving the kernel heap
 	  and everything else nearly all of physical memory.
 
+config STADIUM_WORD_HEAT_QUANTUM
+	int "Q48.16 heat quantum moved per word touch/starter-grant (STADIUM_WORD_HEAT_QUANTUM)"
+	default 2048
+	help
+	  FABRIC.md §17.7 (item 4.1): the fixed Q48.16 amount transferred between
+	  a VM's Stadium reservoir and a word patron's cell on every touch
+	  (already-resident) or starter-grant admission attempt (non-resident,
+	  Option B). Q48_ONE is 65536; the default of 2048 is Q48_ONE divided by
+	  HOTWORDS_CACHE_SIZE (32) -- the population of the cache mechanism this
+	  item retires under __STARKERNEL__ -- so roughly 32 words could hold a
+	  "fully loaded" starter share at once, matching the old cache's slot
+	  count. An untuned placeholder, not a derived optimum: real tuning is
+	  DoE work (item 5.1), same treatment as STADIUM_MEMORY_PERCENT above.
+
+config STADIUM_WORD_COOL_RATE_Q48
+	int "Q48.16 fraction of resident heat removed per tick (STADIUM_WORD_COOL_RATE_Q48)"
+	default 21845
+	help
+	  FABRIC.md §17.7 (item 4.1): redirects Loop #3's decay shape onto
+	  Stadium word-patron heat instead of discarding it -- cooled heat
+	  returns to the VM's reservoir rather than vanishing, so this must be a
+	  fraction of the patron's OWN current heat removed per elapsed tick
+	  (unit-safe for a conserved share of 1.0), not a flat per-tick amount
+	  the way execution_heat's own decay works. Default reuses
+	  INITIAL_DECAY_SLOPE_Q48's numeric value (21845, ~1/3) reinterpreted as
+	  this fraction -- the existing inference engine converged on that
+	  magnitude for the analogous cooling purpose on execution_heat, so it
+	  is a reasonable starting point, not the same quantity. Untuned
+	  placeholder: real tuning is DoE work (item 5.1).
+
 endif # STARFORTH_VARIANT_KERNEL
 
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