feat(v4.0.0): hosted golden model, single node

First code for StarForth v4 (JUSTIFICATION.md section 10, step 1): one node
of the 32-instruction core as a C99 model, with cell width as a build
parameter.

- Node: P, A, B, F18 circular stacks (10 and 9 deep, D-2), word-addressed
  memory (D-1), 5% guard bands on every bounded list.
- Instruction word: six 5-bit slots in 32 bits at every cell width.
- Executor: all 32 opcodes of DECOMPOSITION.md 1.3. Cell arithmetic wraps
  explicitly; no signed overflow or implementation-defined shift.
- Heat: per-opcode and per-call-target counters and the anti-clock, driven
  by instruction retirement (1.4, D-6 interim).
- Slot packer and runner for tests, and a reference unsigned multiply in
  plain C99 with no 128-bit type.

Tests run at 32- and 64-bit cells, and under ASan and UBSan. They cover
every opcode and execute the first section 4 definitions (NIP SWAP OR
NEGATE ROT 0< 0= 2DUP - U<) against the C operation each stands for.

UM* as written in section 4 is exact only while u1 <= 2^(n-2). Two known
failing cases are pinned in test_foundation.c until it is rewritten.

DECOMPOSITION.md: record D-9, the instruction word is 32 bits at every
cell width (ruled 2026-10-02).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/src/testcode.c

@@ -0,0 +1,20 @@
+/* testcode.c -- run a word on a node, for tests.  See testcode.h. */
+#include "v4/testcode.h"
+
+long v4_test_call(v4_node *n, v4_exec_state *es, v4_heat *h,
+                  v4_cell entry, long max_words)
+{
+    long words = 0;
+
+    v4_rstack_push(&n->rs, V4_TEST_HALT);
+    n->p = entry;
+    while (n->p != V4_TEST_HALT) {
+        if (words >= max_words) return -1;
+        /* A golden model must not index outside its own memory on a runaway
+         * P; node.h leaves out-of-range addressing undefined. */
+        if (n->p < 0 || (v4_ucell)n->p >= V4_NODE_WORDS) return -1;
+        (void)v4_exec_step_word(n, es, h);
+        words++;
+    }
+    return words;
+}