feat(stdlib): add Grid.* — tile geometry + A* pathfinding over the tilemap (#24)

Grid.* operates on the Map tilemap (Map.size/Map.row): a cell is passable unless
it is out of bounds or holds the caller's `wall` tile (a char code, e.g. '#'), so
any impassable glyph works. Everything is integer and deterministic.

  - Grid.line(x0,y0,x1,y1) -> []Cell        Bresenham line cells (LOS/raycast base)
  - Grid.blocked(x,y,wall) -> bool          the shared passability test
  - Grid.line_of_sight(x0,y0,x1,y1,wall)    unobstructed straight line?
  - Grid.flood(x,y,wall) -> []Cell          4-connected reachable region (BFS)
  - Grid.a_star(x0,y0,x1,y1,wall) -> []Cell shortest 4-connected path (A*,
                                            Manhattan heuristic), empty if unreachable

The engine (runtime/native/grid.ludic, ~150 lines of Ludic, C-free) is spliced
into a game via core.ludic since it reads the tilemap runtime; returned Cell
slices are ordinary Ludic slices (`len` / `[i]`; each cell has `.x` `.y`).
Pathfinding lives under Grid rather than a `Path` namespace — that name is
already the filesystem-paths library (#10).

Verified against Python references: a 1500-case fuzzer over random maps agrees
exactly on A* path length (optimal, == BFS), flood-fill count, and line-of-sight.
examples/library/grid.ludic asserts the behaviour and is wired into `x test`
(now 61 passed); docs: a Grid section + 5 per-symbol pages, inventory/coverage
green. Seed reseeded; the C-free bootstrap fixpoint holds.

Scope: this lands the Grid.*/pathfinding half of #24. The ECS Query.* helpers
(count/first, and nearest/within which want a runtime spatial index) remain the
tracked follow-up the issue calls out as blocked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/inventory.json

@@ -501,5 +501,12 @@
     "regex-start",
     "regex-end",
     "regex-ok"
+  ],
+  "grid": [
+    "grid-line",
+    "grid-blocked",
+    "grid-line_of_sight",
+    "grid-flood",
+    "grid-a_star"
   ]
 }

tools/x/test.ludic

@@ -103,6 +103,7 @@ function cmd_test() -> int {
   feat_case("library/uuid", "", "1 2 3 4 5 6 7 8 9 10", "uuid.ludic (Uuid v4/v7 format, version/variant, parse/equals)")
   feat_case("library/noise", "", "1 2 3 4 5 6 7 8 9 10 11", "noise.ludic (Noise value/perlin/simplex/fbm/cellular determinism + range)")
   feat_case("library/regex", "", "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18", "regex.ludic (Regex match/find/groups/classes/quantifiers/replace + linear-time safety)")
+  feat_case("library/grid", "", "1 2 3 4 5 6 7 8 9 10 11 12 13", "grid.ludic (Grid line/flood/line_of_sight + A* pathfinding over the tilemap)")
   feat_case("library/logging", "", "0 5 2 1", "logging.ludic (Log levels, set_level/level threshold, structured fields)")
   # Os known-folders/arch and Fs.list read the BSD utsname/dirent layout, so
   # their asserted values are macOS-specific; skip off Darwin (see is_darwin).