refactor(impeeler): rename package from impeller_model

[timcreatedit]

Summary

• Rename `packages/impeller_model` → `packages/impeeler` (name is free on pub.dev)
• `impeeler` = impeller + peel: the tool peels a frame apart into render passes, and the re-implementation is a thin peel of the engine — which also signals the result is an estimate
• Public API: `ImpellerModelBinding` → `ImpeelerBinding`; library entry point `impeller_model.dart` → `impeeler.dart`
• Default report output dir: `test/.impeller_model/` → `test/.impeeler/`
• Skills renamed: `impeeler-gpu-cost`, `impeeler-frame-demand`, `impeeler-gpu-profiling`, `impeeler-engine-upgrade`
• README intro updated with the peel framing; CI job, slides link, and all docs updated

Test plan

• `flutter analyze` clean in `packages/impeeler` and `packages/impeeler/validation`
• `flutter test` in `packages/impeeler`: all 39 tests pass

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