feat(coordinator): add Swarm (PIBT+LNS+BVC) planner and more

[BrokenHypocrite]

Swarm planner, trajectory playback, and Play Chess mode

Adds a complete, collision-free multi-robot planner to the coordinator, plus the
features to use it. All additive: existing planners and the wave-dispatch flow are
unchanged unless the new options are selected.

1. Swarm planner (planning/swarm/, planning/swarm_planner.py)

New planner registered as "Swarm (PIBT+LNS)". Self-contained (standard library
plus planning.base_planner and config only).

• PIBT (priority inheritance with backtracking) plans the choreography on an
  8-connected cell grid. Complete by construction: where EnhancedConflictPlanner
  gets stuck, PIBT makes boxed-in pieces step aside and flow back.
• MAPF-LNS refines the plan for lower total travel (fewer and shorter moves).
• Buffered Voronoi Cells execute it as continuous, collision-free motion.
• plan_moves() forward-simulates the whole pipeline and returns each piece's
  actual sampled trajectory as wave-ordered MoveCommands, so the open-loop robots
  replay a provably collision-free path.
• Optimal interchangeable-piece assignment (any pawn to any pawn square, etc.) and
  in-bounds waypoint clamping.

2. Smooth trajectory playback (simulation/simulator.py)

• New play_trajectory() advances all pieces in lockstep along the synchronized
  trajectory (continuous, arc-like, collision-free), then rotates each to its
  commanded final heading.
• Used automatically for planners that set produces_trajectory (added to
  base_planner.py). The existing rotate/translate wave dispatch is untouched for
  the discrete planners.
• Wiring in main_window.py and path_planning_tab.py routes trajectory planners
  to the player in simulator mode.

3. Play Chess mode (gui/tabs/play_chess_tab.py, gui/chessboard_widget.py)

New tab. Rules via python-chess.

• Click a piece to highlight its legal destination squares.
• Click a legal square to execute the move with the selected planner: captures go
  to the graveyard, castling moves the king and rook, en passant clears the
  bypassed pawn, and promotion auto-queens via the spare queen.
• Turn indicator, SAN move list, check/checkmate/stalemate.
• Hot-seat (no engine opponent).

4. Board size set to 57.15 mm / 2.25 in (config.py)

Changed BOARD.SQUARE_SIZE_MM from 50 to 57.15 mm for two reasons:

• Standard FIDE/USCF tournament square.
• Clearance for the motion planner. A MiniBot is about 31 mm across, so at 50 mm
  the gap between pieces on adjacent squares is only about 19 mm and the
  buffered-Voronoi routing deadlocks or detours often. At 57.15 mm (about a 26 mm
  gap) dense resets stay collision-free and efficient.

All piece coordinates derive from SQUARE_SIZE_MM, so they scale together. The
rationale is documented in config.py and planning/swarm/README.md.

Dependencies

Adds python-chess (see requirements.txt, alongside pyqt6 and pyserial).

Tests

Headless self-tests, run from firmware/MiniBot_Coordinator/:

• python planning/swarm/_selftest.py: plan solved, wave-ordered, collision-free.
• python planning/swarm/_selftest_playback.py: playback completes, stays
  collision-free through interpolation, motion continuous, headings reached.
• python gui/tabs/_selftest_chess.py: normal, capture, castle, en passant, and
  promotion produce correct targets and dispatch through the planner.

Limitations

• Promotion auto-queens via one spare queen per side (no underpromotion UI; a
  second same-side promotion logs a message and keeps the logical board correct).
• Play Chess "New Game" snaps to the start (it does not animate the reset).
• Real-hardware (serial) mode still sends discrete position commands; the smooth
  player is a simulator-playback feature.

[BrokenHypocrite]

This PR mostly just adds an additional path planning algorithm and a way to test it in a Play Chess mode. The one thing I did change is the board's cell size, from 50mm to 57.15mm. There is a massive difference in how easily the planner can route pieces with this change and 2.25in (57.15mm) is a nice size imo since it is tournament size. There is still more improvements that can be made to this planner but I believe it is a good starting point. If you have any questions you can reach me at brokenhypocrite@gmail.com or on discord, username: brokenhypocrite