fix(display): preserve stage aspect ratio and project-colored letterboxing

[joeykchen]

Separate the fitted logical content size from the Web host canvas and replace three stretch bindings with one SetStretch call. Use KEEP for Native and Web without an intermediate IGNORE toggle, so resize preserves stage proportions and camera/input coordinates. Pin the matching Godot letterbox-color fix in a new, unpublished runtime 3.0.1 candidate without changing the published 3.0.0 snapshot.

Parent: refactor/godot-integrated at 45fa2090c033. Dependency-stage merge order: 23/23.

This draft targets dev, so its GitHub diff includes prerequisite changes. The parent SHA above pins the historical review baseline; the current parent branch may already contain this topic. Topic-only diff. Merge after #1925 and the matching engine change goplus/godot#336.

Prior integrated validation: Matching Native/Web builds, generation and relevant Go/Node tests passed. Window-order comparisons covered 576 combinations; Web DPR 1/2 resize checks kept the same visible world and white margins. Native OpenGL/Metal color checks passed. One initial Stop/Start run logged a destroy_sprite warning; an unchanged headless rerun passed, so this PR does not claim to fix that separate cleanup warning. The current update only shortens commit messages; all source trees match the validated history. Validation record. Current diff check also reports an existing extra EOF blank line in the Web ABI test stub.