Add --isolated for per-launch process isolation

[irvingouj@Devolutions (irvingoujAtDevolution)]

Adds --isolated: that launch gets its own process, never forwards to a running instance, and never becomes the broker for later ones.

RDM needs this to swap its bundled wt-distro for dt. It starts one terminal per entry, each with its own env vars (secrets included), WT_BASE_SETTINGS_PATH and WT_PARENT_WINDOW_HANDLE. With the broker, the second launch forwards to the first process and exits, so entry B's shell gets entry A's env and settings, and the parent HWND is dropped. RDM gets around this today with WT's compatibility.isolatedMode (WindowManager.cpp), but WT removed that setting in 1.23 (microsoft/terminal#18215). DHowett floated bringing it back as wt --isolated in microsoft/terminal#19468.

Why a flag and not the old setting: a dt that doesn't know the setting silently ignores it and leaks, while an unknown flag fails the launch (dt: Unknown command '--isolated'.). It's also per launch, and nothing has to be read from settings.json before the election.

With --isolated:

• the host and dt.exe skip forwarding, election and the endpoint file, so a normal launch later can't land in an isolated process either
• persisted layouts aren't restored or saved, same as WT's ShouldUsePersistedLayout
• -w use-existing / window ids exit with 3

Testing

• Cli, App, Settings, Broker, Compatibility, Settings.Editor and UI tests pass; NativeAOT win-x64 publish has no warnings
• Repro: start A, then B, each with a different RDM_SECRET and settings dir. Without the flag, B's shell is a child of A and prints A's secret. With --isolated, each launch keeps its own process, secret and settings dir. Checked on Debug and AOT builds, through both the host and dt.exe. A later launch without the flag also stays out of A.

Not tested yet: HWND embedding inside RDM, macOS/Linux.

[Copilot]

Copilot review overview

🟡 Changes recommended

Isolation is lost during serialization and silently ignored by the in-window command-line path.

Review effort: Balanced
Findings: 2

Open (2)

• Serialize the Isolated property for invocation round trips · New
• Reject --isolated in in-window command-line parsing · New

What changed in this PR

Adds per-launch process isolation, preventing broker participation and persisted-layout reuse.

Changes:

• Adds and documents the --isolated CLI option.
• Skips broker forwarding/election for isolated launches.
• Disables automatic persisted-layout restoration and saving.

File	Description
README.md	Documents isolated launches.
src/​Devolutions.Terminal/​Program.cs	Skips broker setup and forwarding.
src/​Devolutions.Terminal/​App.axaml.cs	Propagates isolation into routing.
src/​Devolutions.Terminal/​TerminalWindowRouter.cs	Passes isolation to windows.
src/​Devolutions.Terminal.Cli/​Program.cs	Launches an isolated host directly.
src/​Devolutions.Terminal.Cli/​CliParser.cs	Parses and advertises --isolated.
src/​Devolutions.Terminal.Cli/​CliContracts.cs	Adds isolation to invocation data.
src/​Devolutions.Terminal.App/​Views/​MainWindow.axaml.cs	Gates persisted-layout behavior.
src/​Devolutions.Terminal.App/​Models/​TerminalLayoutStateStore.cs	Centralizes the layout decision.
tests/​Devolutions.Terminal.Cli.Tests/​CliParserTests.cs	Tests option parsing.
tests/​Devolutions.Terminal.App.Tests/​LayoutDescriptorTests.cs	Tests isolated layout behavior.

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