feat(chess): starting-layout foundation (Phase A)

Introduces a pluggable StartingLayout abstraction so the engine can
open from positions other than FIDE without per-caller special casing.

- layouts/{types,registry,index}.ts: StartingLayout + LAYOUT_REGISTRY,
  mirroring the PRESET_REGISTRY / PIECE_TYPE_REGISTRY pattern.
- starting-position.ts: CLASSIC_LAYOUT + applyLayout(session, layout)
  as the parametrized spawn path. generateStartingPosition stays as
  a thin back-compat wrapper so no existing call site changes.
- layouts/{classic,empty}.ts: first two premades registered via
  side-effect imports from the barrel.
- engine.ts: ChessEngine constructor now accepts either legacy
  (activePresets) positional or new options-bag form
  ({ activePresets?, layout? }). Detection uses a method-shape
  probe rather than instanceof so both overload forms compose
  cleanly under strict TS.
- Tests: 11 new tests in starting-position.test.ts + engine-presets
  cover applyLayout ordering, hasMoved pre-revocation, empty
  layout, classic equivalence, options-bag equivalence with legacy.

Also lands the full execution plans for starting-layouts and
rule-variants under .sisyphus/plans/ (both Momus-reviewed OKAY).

941 tests passing; bun run check clean.

.sisyphus/boulder.json

@@ -35,7 +35,10 @@
     "ses_262ddfb93ffeMGkZK2rspryFfX",
     "ses_262998e6fffeRY6BJVTKB7Jtwb",
     "ses_26299a749ffe3jnwQzrfJ9g1Wc",
-    "ses_261e4ca30ffexBShmvjEhVSHRy"
+    "ses_261e4ca30ffexBShmvjEhVSHRy",
+    "ses_25d474e64ffeuPnRfi4X9al43a",
+    "ses_25d39bf44ffeY9WOIQgNCijBt0",
+    "ses_25d319ba5ffe37R9eKUEq95w7o"
   ],
   "plan_name": "rete-rules-engine",
   "agent": "atlas"