Fix Linux external texture integration

[nmfisher]

Summary

• preserve absolute Linux paths when resolving thermion_dart file URIs
• expose ThermionTextureBootstrap as public API
• vertically flip Linux OpenGL textures for Flutter coordinates
• add regression tests and Linux CI coverage

Verification

• flutter analyze passes
• relevant thermion_flutter tests pass
• verified live in Face Animation Studio on Linux/OpenGL

[nmfisher]

Split into focused PRs:

• fix: absolute thermion_dart package URI resolution on Linux #296 — Linux package URI/build fix
• fix: (flutter) Linux OpenGL external texture orientation #297 — Linux OpenGL texture orientation
• refactor: add ThermionWidget.create constructor for texture bootstrap on Linux  #298 — public texture bootstrap API