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-blocked.md

@@ -0,0 +1,26 @@
+---
+id: grid-blocked
+name: Grid.blocked
+category: grid
+kind: namespace-method
+tokens: Grid.blocked
+sig: Grid.blocked(x, y, wall) -> bool
+tip: True if the cell is out of bounds or holds the wall tile.
+order: 2
+ns: Grid
+member: blocked
+---
+
+Whether <code>(x, y)</code> blocks movement — <code>true</code> if it is out of bounds or its tile equals <code>wall</code>. The passability test the other Grid functions share.
+
+Parameters:
+- `x`, `y` — the cell
+- `wall` — the impassable tile char, e.g. `'#'`
+
+```ludic
+program Demo {
+  handler H phase Update {
+    if Grid.blocked(0, 0, '#') { print(1) }
+  }
+}
+```