Implement Specctra DSN parser with typed board model

Ports the read path of FreeRouting's io/specctra/parser package to a
Java-free Python implementation:

- tokenizer: S-expression lexer mirroring SpecctraFileDescription.flex
  (comments, quoted strings, the string_quote IGNORE_QUOTE directive,
  case-insensitive keywords, and hash-prefixed names)
- sexp: nested S-expression tree builder
- shapes: rect/circle/polygon/path plus area-with-holes scopes
- model: typed dataclasses for layers, padstacks, images, placements,
  nets, net classes, rules, keepouts
- reader: recursive-descent scope readers producing a DsnBoard, one
  _read_* function per FreeRouting read_scope method

46 pytest cases cover the tokenizer, tree, shapes, and end-to-end
parsing against hand-crafted fixtures modeled on FreeRouting's own
test DSN files. Parses 90 of 91 upstream fixtures (the one failure is
a binary OLE file, not text DSN).

tests/dsn/fixtures/empty_board.dsn

@@ -0,0 +1,26 @@
+(pcb freeroute-empty.dsn
+  (parser
+    (string_quote ")
+    (space_in_quoted_tokens on)
+  )
+  (resolution um 10)
+  (unit um)
+  (structure
+    (layer F.Cu
+      (type signal)
+      (property
+        (index 0)
+      )
+    )
+    (layer B.Cu
+      (type signal)
+      (property
+        (index 1)
+      )
+    )
+    (boundary
+      (path pcb 0  186690 -107950  129540 -107950  129540 -57150  186690 -57150
+            186690 -107950)
+    )
+  )
+)