Clear stale QAM tab descriptor on reinjection

[onehoon]

Summary

• Fix the stale global QAM tab descriptor identified in PR Recover Steam native QAM control interaction #515 follow-up validation.
• Clear state.addonTabDescriptor at the start of a new install() generation so a script upgrade cannot reuse a descriptor that closes over old React/native components.
• Clear the descriptor during uninstall() and teardown state reset.
• Add a contract test covering install-generation reset and both uninstall cleanup paths.

Scope

• This is a narrow follow-up to Recover Steam native QAM control interaction #515 and is based on the latest main, where Recover Steam native QAM control interaction #515 is already merged.
• No changes to QAM discovery, QuickSettingsPanel, Runtime, bridge admission, Full 1902 lifecycle, VIIPER, PID, HidHide, Overlay, or PR2 tab topology.

Validation

• node --check src/SteamInputAddonforClaw.QamHost/Frontend/qam.js — passed.
• dotnet build SteamInputAddonforClaw.slnx --configuration Debug --no-restore — passed, 0 warnings, 0 errors.
• dotnet build SteamInputAddonforClaw.slnx --configuration Release --no-restore — passed, 0 warnings, 0 errors.
• Focused QAM contract tests — passed, 29/29.
• Full Release test suite — passed, 2,737/2,737, 0 failed, 0 skipped.
• git diff --check — passed.

Manual validation

• The physical MSI Claw/current Steam GamepadUI acceptance pass remains required and was not available in this environment. After deployment, verify that reinstall/reinjection produces QAM page state: Page=Device ... and actual ToggleField/SliderField mutation logs without reusing a previous descriptor.