starkernel: item 3.1 -- the Stadium cell and header

Punch list §25 item 3.1 complete.

Defines StadiumPatronHeader and StadiumContinuationCell in new
include/starkernel/vm/stadium.h, unioned as StadiumCell per §3's
closed two-valued union. src/starkernel/vm/stadium.c added to
Makefile.starkernel's LOADER_EXTRA_SRCS/KERNEL_EXTRA_SRCS so the
header's compile-time size checks are actually compiled, not merely
included by something that never builds.

Discriminator ruled an external side bitmap (Captain Bob), not a
header field -- amended into §3 and §23.3 before this code was
written. Item 3.1 declares the bitmap's purpose/indexing in a comment
only; allocating it is item 3.2's scope.

Both cell shapes counted for real at exactly 64 bytes with zero
compiler-inserted padding (three C99-portable negative-array-size
assertions -- no _Static_assert, this project targets C99). Header
matches §23.3's original 32+32 split unchanged, since the
discriminator moving outside the cell left nothing to compete for that
space. Continuation cell matches item 1.12's 4+60 figure unchanged for
the same reason.

Verified the size assertion is actually live: broke it to 63,
confirmed the build failed with the expected negative-array-size
error, restored it, confirmed a clean compile.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-2.2 baseline. Confirmed stadium.o present in both obj/loader/vm
and obj/kernel/vm post-build on amd64, closing the gap the item-2.2 WIP
exposed (an uncompiled header proves nothing).

Left open, not fabricated: §23.4 #2 ("does a typical message fit in
one cell") is unanswerable today -- no message patron struct exists
anywhere in this tree yet.

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

src/starkernel/vm/stadium.c

@@ -0,0 +1,33 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+ */
+
+/**
+ * stadium.c - Translation unit for the Stadium cell definitions (item 3.1).
+ *
+ * Exists so stadium.h's compile-time size assertions are actually compiled
+ * on all three architectures, not merely included by something that never
+ * builds. No behavior yet -- item 3.2 onward adds the array, dispatch, and
+ * ranking.
+ */
+
+#include "starkernel/vm/stadium.h"