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/minimal.txt

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