nav, physics: Recast's and Detour's memory counted (nav_heap_bytes / _peak / _allocs, a counted allocator registered at load, tile bytes staged from it); package tests that crossing the map and back holds the native bytes

The user's walk showed the footprint rising outside the Ludic heap. Jolt's bytes were already counted;
the navmesh's were not, so nothing could see them. Now:
- ludic.nav nav_heap_test: fifty crossings of a four-tile map by the resident index hold the tiles in and
  the native bytes to the first crossing's; a thousand crowd walkers in and out grow nothing; a reset
  gives back every byte the mesh took.
- ludic.physics cross_heap_test: a 1 km map of 64 m chunks kept to a ring round a player crossing
  corner to corner and back, each chunk a heightfield, twelve owned posts and six owned scaled hulls
  as the game makes them: six more crossings hold the bodies, the shapes and Jolt's bytes and peak
  exactly. (A post made as an offset of a cylinder, with only the offset owned, leaked the cylinder
  every time: the game's solid_pillar owns its cylinder directly.)

lib/macos-arm64 rebuilt; lib/windows-x64 needs native/build.sh run on the PC for the new exports.

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

packages/ludic.nav/native/shim/nav_alloc.inl

@@ -0,0 +1,42 @@
+// nav_alloc.inl - Recast's and Detour's allocations counted apart from the game's (plan 25), as
+// ludic.physics counts Jolt's: libc's malloc underneath, each block carrying its size in a 16-byte
+// header, so the tiles, queries and crowds are measured exactly. Registered when the library loads.
+#include <atomic>
+
+namespace {
+constexpr size_t kNavHead = 16;
+std::atomic<long long> g_nav_live{0}, g_nav_peak{0}, g_nav_allocs{0};
+
+void *nav_heap_alloc(size_t n) {
+  char *b = static_cast<char *>(std::malloc(n + kNavHead));
+  if (!b) return nullptr;
+  std::memcpy(b, &n, sizeof n);
+  g_nav_allocs.fetch_add(1, std::memory_order_relaxed);
+  long long now = g_nav_live.fetch_add((long long)n, std::memory_order_relaxed) + (long long)n;
+  long long p = g_nav_peak.load(std::memory_order_relaxed);
+  while (now > p && !g_nav_peak.compare_exchange_weak(p, now, std::memory_order_relaxed)) {}
+  return b + kNavHead;
+}
+void nav_heap_free(void *p) {
+  if (!p) return;
+  char *b = static_cast<char *>(p) - kNavHead;
+  size_t n;
+  std::memcpy(&n, b, sizeof n);
+  g_nav_live.fetch_sub((long long)n, std::memory_order_relaxed);
+  std::free(b);
+}
+void *nav_dt_alloc(size_t n, dtAllocHint) { return nav_heap_alloc(n); }
+void *nav_rc_alloc(size_t n, rcAllocHint) { return nav_heap_alloc(n); }
+
+struct NavHeapRegister {
+  NavHeapRegister() {
+    dtAllocSetCustom(nav_dt_alloc, nav_heap_free);
+    rcAllocSetCustom(nav_rc_alloc, nav_heap_free);
+  }
+} g_nav_heap_register;
+}  // namespace
+
+// bytes Recast and Detour hold now, the most they ever held, and how many blocks they asked for
+NAV_SHIM long long nav_heap_bytes(void) { return g_nav_live.load(std::memory_order_relaxed); }
+NAV_SHIM long long nav_heap_peak(void) { return g_nav_peak.load(std::memory_order_relaxed); }
+NAV_SHIM long long nav_heap_allocs(void) { return g_nav_allocs.load(std::memory_order_relaxed); }