Add text-DSL parser with line-numbered errors

Two-pass-free line-based scanner. Each directive carries its source
line so downstream validation errors can be attributed correctly.
Parser checks syntax only — atlas/tile resolution and bounds checks
are deferred to the builder so each tier has one responsibility.

tests/fixtures/dsl/error_oob.txt

@@ -0,0 +1,4 @@
+size 4 4
+atlas atlas_a
+layer surface
+  set 10 10 atlas_a:grass

tests/fixtures/dsl/full.txt

@@ -0,0 +1,16 @@
+# full demo with comments and rotation
+id full_demo
+size 4 4
+atlas atlas_a
+atlas atlas_b
+
+layer surface
+  fill 0 0 3 3 atlas_a:grass
+  set 1 1 atlas_a:stone rot 2
+
+layer wall
+  set 0 0 atlas_b:water rot 1
+
+roof
+  set 1 1 1
+  set 2 2 0

tests/fixtures/dsl/minimal.txt

@@ -0,0 +1,4 @@
+size 4 4
+atlas atlas_a
+layer surface
+  set 0 0 atlas_a:grass