[codex] Fix label selection across deferred viewports

[yay]

What changed

• Keep label text-selection state separate for each viewport.
• Route pass lifecycle and label painting through the current ViewportId.
• Drop inactive per-viewport state after its pass.
• Add a regression test that verifies an unrelated viewport pass cannot clear a child viewport's
  selection, while the owning viewport still clears selections whose labels disappear.

Why

Issue #4758 identified that deferred viewports need independent label-selection state. PR #4760
fixed it by keying the temporary state by viewport. The plugin refactor in PR #7385 moved that
state into one context-wide LabelSelectionState, which accidentally removed the viewport
isolation. A pass in another viewport then fails to encounter the selected widgets and clears the
selection.

This restores the behavior of #4760 within the current plugin architecture. Applications do not
need any special handling.

Validation

• cargo test -p egui
• cargo clippy -p egui --all-targets --all-features -- -D warnings
• cargo test -p egui_tests test_atom_selectable_text_can_be_copied
• Steady integration: cargo check -p steady --release
• Steady integration: cargo test -p steady --release gui::note_window
• ./scripts/check.sh passes formatting, lint, docs, debug/release/WASM clippy, all-target and
  all-feature tests, and doc tests. It then reaches an unrelated failure already present on
  upstream/main: maybe_attach_inspection_plugin is dead code in the no-default-feature
  egui_demo_app check.

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8242-codex/fix-deferred-viewport-label-selection

View snapshot changes at kitdiff