[2/5] Complete the native v2 action and protocol surface

[cm2435-hcomp]

Why

This is the second PR in the background-driver v2 native stack. It completes the protocol and action surface on top of the typed controller foundation so the real-host workflows can exercise one coherent Rust implementation.

What changed

• Wires the v2 JSON protocol through the server and CLI.
• Adds the complete requested action vocabulary: semantic click, targeted point click, scroll, key/chord, text insertion, set value, text selection, drag, and secondary actions.
• Adds typed route decisions, focused-element validation, modifier cleanup, post-action settlement, and partial-delivery evidence.
• Adds the protocol fixture used by the Python client boundary tests.
• Refuses background cells that have an implementation but no published real-host recipe.

Stack

• Depends on [1/5] Add the background window driver v2 foundation #1.
• This PR corresponds approximately to implementation plans 6 through 9.
• The following PR replaces the original experimental macOS click/focus assumptions with the current signed Swift helper’s route.

Validation

Validated at the final stack head:

• cargo test -p cua-driver-core: 148 tests passed, 0 failed.
• cargo test -p platform-macos: 197 tests passed, 0 failed, 1 doc test ignored.
• git diff --check: passed.

Repository-wide formatting remains red on unrelated 0.7.1 files and is not rewritten here.

Known limits

• press_key is implemented but deliberately unpublished in every framework cell pending an exact keyboard recipe.
• Drag, delta scroll, menu dismissal, Catalyst, and several framework cells remain unproved on the real host.
• The macOS action implementation in this commit still reflects the earlier Chromium-first experiment. The next PR contains the Swift-reference correction and accepted Calculator evidence.

[github-actions]

Linux visual regression artifacts

Matrix jobs now run independently. Download visual artifacts from this workflow run.
Each background-GUI job uploads a .gif of the interaction plus two annotated PNGs (<app>.png raw, <app>-atspi.png with AT-SPI element boxes); the cua-driver-linux-som-overlays artifact adds <app>-som.png cua Set-of-Marks overlays:

• cua-driver-linux-cursor-click-gif
• cua-driver-linux-background-terminal-gif
• cua-driver-linux-parallel-drag-xserver
• cua-driver-linux-background-gui-chromium
• cua-driver-linux-background-gui-tk
• cua-driver-linux-background-gui-gtk3-gedit
• cua-driver-linux-background-gui-gtk3-mousepad
• cua-driver-linux-background-gui-gtk3-scite
• cua-driver-linux-background-gui-gtk4-characters
• cua-driver-linux-background-gui-qt5-manuskript
• cua-driver-linux-background-gui-qt5-klog
• cua-driver-linux-background-gui-qt5-openambit
• cua-driver-linux-background-gui-qt6-kate
• cua-driver-linux-background-gui-qt6-kcalc
• cua-driver-linux-background-gui-qt6-okular
• cua-driver-linux-background-gui-qt6-qownnotes
• cua-driver-linux-background-gui-electron-zettlr
• cua-driver-linux-background-gui-electron-joplin
• cua-driver-linux-background-gui-electron-logseq
• cua-driver-linux-som-overlays

Open workflow run and download artifacts