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>

docs/language/grid/grid-a_star.md

@@ -0,0 +1,28 @@
+---
+id: grid-a_star
+name: Grid.a_star
+category: grid
+kind: namespace-method
+tokens: Grid.a_star
+sig: Grid.a_star(x0, y0, x1, y1, wall) -> []Cell
+tip: The shortest 4-connected path between two cells (A*), or an empty list.
+order: 5
+ns: Grid
+member: a_star
+---
+
+Returns the shortest path from <code>(x0, y0)</code> to <code>(x1, y1)</code> over passable (non-<code>wall</code>) cells, 4-connected with uniform step cost, as a <code>Cell</code> slice from start to goal inclusive — an <strong>A*</strong> search with a Manhattan heuristic. Empty if the goal is unreachable (or start/goal is a wall). (Named under <code>Grid</code> rather than <code>Path</code>, which is the filesystem-paths library.)
+
+Parameters:
+- `x0`, `y0` — the start cell
+- `x1`, `y1` — the goal cell
+- `wall` — the impassable tile char, e.g. `'#'`
+
+```ludic
+program Demo {
+  handler H phase Update {
+    let path = Grid.a_star(1, 1, 20, 12, '#')
+    if len(path) > 0 { print(len(path)) }
+  }
+}
+```