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>

selfhost/backend/emit_call.ludic

@@ -203,6 +203,18 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "end")         { bare = "regex_end";         push(labels, "match"); push(labels, "n") }
     if (meth == "ok")          { bare = "regex_ok";          push(labels, "match") }
   }
+  # Grid.* — tile geometry and pathfinding over the Map tilemap, from
+  # runtime/native/grid.ludic (spliced with core.ludic). `wall` is the impassable
+  # tile char, e.g. '#'. line/flood/a_star return []Cell slices. (Pathfinding
+  # lives under Grid rather than a `Path` namespace — that name is the filesystem
+  # paths library.)
+  if (ns == "Grid") {
+    if (meth == "line")          { bare = "grid_line";          push(labels, "x0"); push(labels, "y0"); push(labels, "x1"); push(labels, "y1") }
+    if (meth == "blocked")       { bare = "grid_blocked";       push(labels, "x"); push(labels, "y"); push(labels, "wall") }
+    if (meth == "line_of_sight") { bare = "grid_line_of_sight"; push(labels, "x0"); push(labels, "y0"); push(labels, "x1"); push(labels, "y1"); push(labels, "wall") }
+    if (meth == "flood")         { bare = "grid_flood";         push(labels, "x"); push(labels, "y"); push(labels, "wall") }
+    if (meth == "a_star")        { bare = "path_a_star";        push(labels, "x0"); push(labels, "y0"); push(labels, "x1"); push(labels, "y1"); push(labels, "wall") }
+  }
   if (bare == null) { perr(`unknown builtin {ns}.{meth}`) }
   reorder_named(e, labels)
   let id = node(E_ID); id.s = bare; e.a = id