Replace hardcoded piece catalog with registry pattern

[francinv]

Piece types are hardcoded via switch/case and arrays in multiple files. Adding a new piece requires touching 7+ files with 16 side-effect touchpoints.

Hardcoded locations:

• PieceFactory.java:26-57 — three separate switch blocks
• SetupPaletteWidget.java:25-31 — PIECE_NAMES / PIECE_COSTS arrays
• SetupBoardCodec.java:20-25, 52-70 — numeric codes + instanceof chain
• GameOverlayManager.java:171-176 — hardcoded promotion names

What to do:

• Create a piece registry that maps type name → class, cost, texture, codec
• PieceFactory, SetupPaletteWidget, SetupBoardCodec, and GameOverlayManager should read from the registry instead of maintaining their own hardcoded lists
• Adding a new piece type should require: one new class + one registry entry + sprite assets

Ref: Architecture doc claims M2 (add piece in <5h with no side effects).