feat: Add input, world snapshot and diff commands to flame_cli

[spydon]

Description

Stacked on #4056. This turns the CLI from an observer into something that can play test a game,
and gives agents a way to compare snapshots that does not rely on eyeballing two images.

• A new InputConnector in flame registers ext.flame_devtools.tap, drag and key. Taps
  and drags are delivered through the game's MultiTapDispatcher and MultiDragScaleDispatcher,
  so they reach TapCallbacks and DragCallbacks components with the same propagation as real
  input, in canvas coordinates (the same as the pixels of a snapshot). Keys go to onKeyEvent of
  a HasKeyboardHandlerComponents or KeyboardEvents game, with press, down and up
  actions. Key names are matched against the LogicalKeyboardKey names without regard to case,
  spaces and underscores (arrowLeft, Arrow Left, space, a, digit1). A missing mixin is
  reported as an invalid parameters error instead of silently doing nothing.
• flame input tap <x,y>, flame input drag <x,y> <x,y> [--steps N] and
  flame input key <name> [--down|--up] call those.
• getGameSnapshot takes world=true or rect=x,y,w,h to render the world directly without the
  camera, so off screen components are visible. Without a rect the image covers the bounding
  rectangle of all the position components in the world. flame snapshot --world and
  --rect expose it. Snapshotting the World component itself used to give a blank 100x100
  image, since World.renderTree is a no-op, it now renders the world's children the same way.
• flame diff <before.png> <after.png> [--output diff.png] [--threshold N] [--exit-code]
  reports the percentage of differing pixels and the bounding rectangle of the change, and can
  write an image with the changes in red on a dimmed copy of the second image. This adds the
  image package as a dependency of flame_cli.

Tests cover the input connector with real components (tap position, drag steps, key down and up
with the pressed set, the missing mixin messages, key name lookup), world bounds and world
snapshots, the diff algorithm and command, and the usage errors of the new commands. Verified
end to end on the flutter-tester device: a tap toggled a component's color and flame diff
reported exactly the 200x100 rectangle at 300,250 with exit code 1, a drag and arrow keys moved
the component, and --world and --rect rendered an off screen component.

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.

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