Parse .kicad_pcb on a real s-expression parser (KiCad 5-10)

[prat96]

What

Replaces the regex .kicad_pcb loader with a real s-expression parser that
reads every KiCad dialect from 5 through 10.

Why

KiCad 8/9/10 dropped the numbered net table and moved references into
(property "Reference" ...). The old regex loader returns 0 nets / 0 pads
on those files, so loading a board directly from disk (the cli and headless
paths) is effectively broken on any modern board.

What's in it

• sexpr.py — a small, dependency-free balanced-paren parser (Python 3.9
  clean) plus byte-exact, idempotent top-level strip helpers.
• file_parser.py — structural pad/net/layer extraction across both dialects:
  legacy numbered-net + (module ...), and modern net-table-free
  (footprint ...)/property with name-only (net "NAME") synthesis.
  Footprint rotation is CCW-positive in KiCad's Y-down frame and back-side pad
  offsets carry no extra mirror — verified pad-exact (≤1 µm) against
  pcbnew 10.0.4. Copper-layer counting excludes Edge.Cuts; design rules are
  merged from a sibling .kicad_pro when present.
• Small fixes ridden along: pad_escape_planner reads drill with a
  drill_size fallback; rich_kicad_interface imports KiCad from kipy
  (matching the rest of the codebase).

Tests

Small, self-contained synthetic boards (no external board files):

• test_sexpr — parser round-trips + idempotent strip helpers.
• test_file_parser — both dialects: legacy numbered-net (module ...), and
  modern net-table-free (footprint ...)/property with name-only net
  synthesis, back-side pad placement, and Edge.Cuts exclusion.

$ python -m pytest tests/test_sexpr.py tests/test_file_parser.py
27 passed

Notes for review

• This replaces a core input path. Happy to gate it behind an issue/discussion
  first if you'd prefer — opened as a PR because the old path returns nothing
  on current KiCad files, so it's arguably a bug fix.
• Pad-transform correctness was validated pad-exact against pcbnew 10.0.4 on
  real multi-layer boards (front/back, rot 0/90/180/270). The shipped tests pin
  the same transform on synthetic boards so the suite stays fixture-free.

Checklist

• Tests pass locally (27 passed on a clean clone)
• No new runtime dependencies (stdlib only)
• Preserves KiCad 5–7 parsing (legacy dialect is tested)
• Google-style docstrings on new code