feat(nav): 17.1/17.3 - ludic.nav over Recast & Detour v1.6.0 (zlib), built here from the pinned tag on the Mac and the PC (the DLL imports KERNEL32 alone): a navmesh per kind of walker from triangles with an area byte each and cylinders nothing stands in, saved and loaded as bytes; the nearest point, a path as corners (partial when the end cannot be reached), whether a straight line stays walkable, a cost per kind of ground. Tests on a hand-built meadow: round a post, over a ford, stopped at a bank, a saved mesh answering alike - Mac and PC

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

packages/ludic.nav/README.md

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+# ludic.nav
+
+A walkable mesh of the world and the ways across it, over
+[Recast & Detour](https://github.com/recastnavigation/recastnavigation) (zlib) built here from a
+pinned tag (phase 17). Uses `ludic.base` and nothing else.
+
+```ludic
+import "ludic.nav"
+```
+
+## The rules it keeps
+
+- **One mesh per kind of walker.** `NAV_PERSON` (0.35 m wide, the hiker's 0.55 m step),
+  `NAV_LARGE` (an elk, a bear, a horse) and `NAV_SMALL` (a hare, a marmot) each have their own,
+  because what a hare slips between a bear walks round. A `NavConfig` says what a mesh is built
+  for: its voxels and its walker's height, radius, step and steepest slope.
+- **Built from triangles, or loaded from a bake.** `nav_build` rasterizes triangles - each with an
+  area byte, 0 not walkable and 1..62 a kind of ground - and marks every cylinder given (a trunk, a
+  post) as somewhere nothing stands; ground steeper than the slope is cleared whatever its area. A
+  game bakes its maps once and loads the bytes (`nav_save_file` / `nav_load_file`): a map is never
+  built at start-up.
+- **A path is corners.** `nav_path` answers the way from one point to another as the corners
+  where it turns, the first the start and the last the end. When the end cannot be reached the
+  path stops as near it as the mesh allows and `nav_partial` says so - a walker goes there and
+  gives up, never through a river. Either end off the mesh is `-1`.
+- **A kind of ground has a cost** (`nav_area_cost`): a trail cheaper than a meadow, scree dearer.
+- **The same question gets the same answer.** Detour is deterministic for the same mesh and the
+  same points, so co-op's order of dice is untouched by asking it.
+
+## API
+
+| | |
+| --- | --- |
+| `NAV_PERSON`, `NAV_LARGE`, `NAV_SMALL`, `NAV_CORNERS` | the kinds of walker, and a path's most corners (256) |
+| `NavConfig { cell, cell_h, height, radius, climb, slope }` | what a mesh is built for (a person by default) |
+| `nav_build(kind, v, nv, t, nt, area, cyl, nc, config) -> bool`, `nav_polygons(kind)` | a mesh from triangles and cylinders (`x, y, z, r, h` each) |
+| `nav_save_file(kind, path)`, `nav_load_file(kind, path)`, `nav_reset()` | a baked mesh written and read; every mesh let go |
+| `nav_nearest(kind, x, y, z) -> bool`, `nav_near_x/y/z()` | the nearest walkable point within a couple of metres |
+| `nav_path(kind, sx, sy, sz, ex, ey, ez) -> int`, `nav_corners()`, `nav_corner_x/y/z(i)`, `nav_partial(kind)` | a way as corners |
+| `nav_straight(kind, sx, sy, sz, ex, ez) -> bool` | does the straight line stay walkable |
+| `nav_area_cost(kind, area, cost)` | how dear a kind of ground is to cross |
+
+## Tests
+
+```bash
+ludic test packages/ludic.nav
+```
+
+A 40 m meadow built by hand: a path round a post, over a river's ford, stopping at the bank of a
+river with none, the nearest point, and a saved mesh answering as the built one did.
+
+## The native library
+
+`native/build.sh` fetches Recast & Detour v1.6.0, checks its SHA-256, and builds Recast, Detour
+and the shim (`native/shim/nav_shim.cpp`) into `lib/<target>/` - the same script on the Mac and on
+the PC (Git Bash, the LLVM installer's clang). `native/LICENSE-recastnavigation` ships with it.