feat!: Migrate flame_forge2d to the Box2D v3 based forge2d

[spydon]

Description

Migrates flame_forge2d (and everything in the monorepo that uses it) to the new forge2d, the
native Box2D v3.1.1 bindings that are currently in review in the stacked chain
flame-engine/forge2d#115 (the native rewrite) and flame-engine/forge2d#116 (web support through a
WebAssembly build). This both prepares the migration and serves as real-world validation of the new
forge2d API while that chain is in review.

flame_forge2d now depends on the published forge2d: ^0.15.0. flame_forge2d stays in the
customer_testing.dart exclusions, because forge2d compiles Box2D from source through the Dart
build hooks and so needs a C toolchain on the flutter/flutter presubmit runner; the reasoning is
documented next to the exclusion.

Core package

• Forge2DWorld steps the world with physicsWorld.step(dt, subStepCount: subStepCount) and then
  dispatches the polled contact and sensor events through the new overridable
  ContactEventsDispatcher (replacing WorldContactListener, since listener interfaces no longer
  exist). It keeps a Dart-side bodies set (upstream no longer exposes one) which the gravity
  setter uses to wake bodies, and exposes the new query API (castRayClosest, castRay,
  castRayAll, overlapAabb) plus forwarding setters for preSolveCallback and
  customFilterCallback.
• ContactCallbacks keeps its familiar beginContact(Object other, Contact contact) shape through
  a new lightweight flame-side Contact class that wraps both contact and sensor events.
• BodyComponent renders from the new Shape.geometry read-back (Circle, Capsule, Segment,
  Polygon; chain segments arrive as Segments), with renderShape/renderSegment and a new
  renderCapsule. fixtureDefs is replaced by shapeSpecs (a list of ShapeSpec, pairing a
  ShapeGeometry with an optional ShapeDef). The default createBody() auto-enables
  contact/sensor events on shapes whose body or shape userData is a ContactCallbacks, since the
  new engine only generates events for shapes that opted in.
• SDK floors raised to Dart >=3.12.0 / Flutter >=3.44.0 (root workspace, melos bootstrap,
  package, and FLUTTER_MIN_VERSION in CI).
• Forge2DGame awaits initializeForge2D() in its onLoad, and Forge2DWorld creates its
  physics world lazily so that this can happen first. Without it every Forge2DGame throws on the
  web, since that call is what loads the Box2D WebAssembly module. Code that creates a world
  outside of a Forge2DGame has to await it itself.
• Tests rewritten against real physics worlds (mocked Fixture/Contact/Manifold are gone) and
  all goldens regenerated. flame's MultiTapDispatcher.handleTapDown annotation changed from
  @internal to @visibleForTesting so the tests can use it without ignores.

Examples and docs

• The examples stories, padracing, and the package example are migrated. The examples for joints
  that no longer exist in Box2D v3 (gear, pulley, rope, friction, constant-volume) and the blob
  example are removed.
• doc/bridge_packages/flame_forge2d/forge2d.md and joints.md are rewritten for the new API
  (including the new filter and wheel joints).

Verification

• flutter test in packages/flame_forge2d (45 tests, compiles native Box2D through build hooks),
  goldens visually inspected.
• dart analyze clean across the whole workspace.
• flutter build web of the examples app confirms the Box2D wasm module is bundled automatically
  at the package asset path.

Notes for the forge2d review (found during this migration)

• Shape.geometry read-back was added upstream during this work and is what makes
  BodyComponent rendering possible without a Dart-side geometry registry.
• There is no upstream way to enumerate a world's bodies, hence the Dart-side set in
  Forge2DWorld.
• Behavior change to be aware of: destroying a body clears its userData registries, so removed
  BodyComponents no longer receive a final endContact for contacts that end due to the
  destruction (the old engine fired those synchronously inside destroy).

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Migration instructions

• Fixture and FixtureDef are gone: create shapes with
  body.createShape(geometry, ShapeDef(...)), where the geometry is a Circle, Capsule,
  Segment, or Polygon. Friction and restitution now live in ShapeDef.material
  (a SurfaceMaterial). body.fixtures becomes body.shapes.
• BodyComponent.fixtureDefs becomes shapeSpecs, a list of ShapeSpec(geometry, [shapeDef]).
  renderFixture becomes renderShape, renderEdge becomes renderSegment, and renderChain
  is gone (chain segments render as segments).
• Shape construction: CircleShape()..radius = r becomes Circle(radius: r, center: c),
  EdgeShape()..set(a, b) becomes Segment(point1: a, point2: b),
  PolygonShape()..setAsBoxXY(w, h) becomes Polygon.box(w, h), and
  ChainShape()..createChain/createLoop becomes body.createChain(ChainDef(points: ..., isLoop: ...)) (chains now require at least four points; the first and last points of an open chain are
  ghost anchors).
• BodyDef.angle becomes BodyDef(rotation: Rot.fromAngle(angle)).
• Contact events are now opt-in per shape: set ShapeDef.enableContactEvents (and
  enableSensorEvents for sensors and their visitors). The default BodyComponent.createBody()
  enables them automatically when a ContactCallbacks is present in the body's or shape's
  userData.
• ContactCallbacks.beginContact/endContact keep their signatures but receive the new flame-side
  Contact (with shapeA, shapeB, bodyA, bodyB, begin-only normal/points, and
  isSensorEvent). preSolve/postSolve are removed: use Forge2DWorld.preSolveCallback with
  ShapeDef.enablePreSolveEvents, and hit events (ShapeDef.enableHitEvents +
  world.physicsWorld.contactEvents.hit) respectively.
• WorldContactListener is replaced by ContactEventsDispatcher, and the contactListener
  parameter of Forge2DGame by contactEventsDispatcher.
• Joints are now created with typed methods and destroyed on the joint:
  world.physicsWorld.createRevoluteJoint(RevoluteJointDef(bodyA: ..., bodyB: ...)) and
  joint.destroy(). The available joints are distance, filter, motor, mouse, prismatic, revolute,
  weld, and wheel; gear, pulley, rope, friction, and constant-volume joints no longer exist. The
  def initialize helpers are gone; anchors are passed as local points
  (body.localPoint(worldAnchor)).
• Queries: world.raycast(callback, p1, p2) becomes castRayClosest/castRay/castRayAll
  (taking an origin and a translation), queryAABB becomes overlapAabb, and clearForces and
  the particle system are removed.
• Body API renames: worldCenter becomes worldCenterOfMass, setAwake(x) becomes isAwake = x,
  getInertia() becomes rotationalInertia, and worldVector(v) becomes rotation.rotate(v).
• Platform requirements: Dart 3.12+ / Flutter 3.44+, a C toolchain when building for native
  platforms, and nothing extra on the web (the Box2D wasm module is bundled automatically).

Related Issues

Depends on flame-engine/forge2d#115 and flame-engine/forge2d#116.