starkernel: item 3.2 -- Stadium boot-time allocation

Punch list §25 item 3.2 complete.

stadium_boot_init() (src/starkernel/vm/stadium.c) sizes the global
cell array at boot from a real memory-budget query rather than a
hardcoded count: pmm_get_stats().free_bytes at the point of
allocation, times the new STADIUM_MEMORY_PERCENT Kconfig symbol
(default 1%), rounded down to whole 64-byte cells. Matches §17.6's
position (b) literally. Also allocates the header/continuation
discriminator bitmap item 3.1 declared but did not allocate. Both are
kmalloc'd and explicitly zero-filled (kmalloc does not zero).

Called from kernel_main.c immediately before sk_vm_bootstrap_parity(),
i.e. before any VM exists (§6). Failure is soft -- logs and continues,
does not halt boot -- matching the existing precedent one line below
it (VM bootstrap parity failure does the same).

Added a "Stadium: N cells (M KB)" boot console line at the allocation
site so the acceptance logs are evidence the array was actually
allocated, not just that the kernel still boots -- the same blind spot
item 3.1's uncompiled-header gap exposed.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-3.1 baseline, and the Stadium boot line confirmed present in all
three serial logs (amd64: 74234 cells/4639 KB, aarch64: 161329
cells/10083 KB, riscv64: 76122 cells/4757 KB).

Not built here, reported per §25.0 rule 3: per-VM free lists (§22.3)
-- granted when Hera assigns quota, not this item's scope.

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

include/starforth_config.h

@@ -73,6 +73,7 @@
 #define STARFORTH_CONFIG_STADIUM_MAX_VM_COUNT_DEFAULT 4
 #define STARFORTH_CONFIG_STADIUM_CONTAINS_DEPTH_MAX_DEFAULT 5
 #define STARFORTH_CONFIG_STADIUM_CAPACITY_TICK_DEFAULT 1000
+#define STARFORTH_CONFIG_STADIUM_MEMORY_PERCENT_DEFAULT 1
 #define STARFORTH_CONFIG_HEARTBEAT_CHECK_FREQUENCY_DEFAULT 256
 #define STARFORTH_CONFIG_HEARTBEAT_WINDOW_TUNING_FREQUENCY_DEFAULT 1000
 #define STARFORTH_CONFIG_HEARTBEAT_SLOPE_VALIDATION_FREQUENCY_DEFAULT 5000
@@ -170,6 +171,10 @@
 #define STADIUM_CAPACITY_TICK STARFORTH_CONFIG_STADIUM_CAPACITY_TICK_DEFAULT
 #endif
 
+#ifndef STADIUM_MEMORY_PERCENT
+#define STADIUM_MEMORY_PERCENT STARFORTH_CONFIG_STADIUM_MEMORY_PERCENT_DEFAULT
+#endif
+
 #ifndef HEARTBEAT_CHECK_FREQUENCY
 #define HEARTBEAT_CHECK_FREQUENCY STARFORTH_CONFIG_HEARTBEAT_CHECK_FREQUENCY_DEFAULT
 #endif

include/starkernel/vm/stadium.h

@@ -34,8 +34,9 @@
 
 #ifdef __STARKERNEL__
 
+#include <stddef.h>
 #include <stdint.h>
-#include "starforth_config.h" /* STADIUM_CONTAINS_DEPTH_MAX, STADIUM_CAPACITY_TICK */
+#include "starforth_config.h" /* STADIUM_CONTAINS_DEPTH_MAX, STADIUM_CAPACITY_TICK, STADIUM_MEMORY_PERCENT */
 
 #define STADIUM_CELL_BYTES 64
 
@@ -103,6 +104,40 @@ typedef char stadium_cell_size_check[(sizeof(StadiumCell) == STADIUM_CELL_BYTES)
 typedef char stadium_contains_depth_configured_check[(STADIUM_CONTAINS_DEPTH_MAX > 0) ? 1 : -1];
 typedef char stadium_capacity_tick_configured_check[(STADIUM_CAPACITY_TICK > 0) ? 1 : -1];
 
+/*
+ * stadium_boot_init - Boot-time allocation (FABRIC.md item 3.2, §17.6 position
+ * (b)). Sizes the global cell array from the memory budget actually observed
+ * at boot -- STADIUM_MEMORY_PERCENT of pmm_get_stats().free_bytes at the
+ * point of the call, rounded down to whole STADIUM_CELL_BYTES cells -- rather
+ * than a hardcoded count. Also allocates the header/continuation discriminator
+ * bitmap item 3.1 declared but did not allocate: one bit per cell, bit set
+ * means the cell at that index is a patron header, clear means continuation
+ * or not yet in use. Both are kmalloc'd (freestanding kernel, no separate
+ * PMM-backed region needed for this) and explicitly zero-filled, since
+ * kmalloc does not zero.
+ *
+ * Must be called after M6 (kmalloc_init) and before any VM is born (§6). Does
+ * not halt boot on failure -- nothing downstream consumes the Stadium yet.
+ *
+ * @return 0 on success, -1 if kmalloc failed for either allocation.
+ */
+int stadium_boot_init(void);
+
+/* stadium_is_initialized - Whether stadium_boot_init() has succeeded. */
+int stadium_is_initialized(void);
+
+/* stadium_cell_count - Number of cells in the array, 0 if not initialized. */
+size_t stadium_cell_count(void);
+
+/* stadium_cells - Pointer to the cell array, NULL if not initialized. */
+StadiumCell *stadium_cells(void);
+
+/*
+ * stadium_header_bitmap - Pointer to the discriminator bitmap declared in
+ * item 3.1, NULL if not initialized. ceil(stadium_cell_count() / 8) bytes.
+ */
+uint8_t *stadium_header_bitmap(void);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_STADIUM_H */