Split Steam runtime and game runner routing

[weter11]

Motivation

• Prevent Proton bootstrap from running against the background Steam host prefix by decoupling which runtime hosts background Steam from which runtime hosts the game.
• Let real Proton trees run games via their proton run script so Proton's bundled protonfixes run naturally, and provide a fallback fixup mechanism for plain Wine/wine-tkg.
• Provide deterministic detection and routing across the full 6-row support matrix and surface clear errors/warnings for unknown runner configurations.

Description

• Added runner classification (RunnerKind) and classify_runner + validate_steam_runtime_runner_path in src/utils.rs, including unit tests exercising Proton/plain-Wine/Unknown cases and the 6-row support-matrix routing decisions.
• Changed background Steam startup in WineTkgRunner::prepare_prefix to never fall back to the game runner: it now resolves a wine-tkg/plain Wine runner or extracts and invokes Proton's bundled wine64 directly for background Steam (bare wine invocation), and surfaces clear LaunchError for unknown/missing cases.
• Added game-runner classification in the game launch path so real Proton with protonfixes uses proton run unchanged, while plain Wine / Proton-without-protonfixes uses bare wine and runs the new Rhai fixup layer; merged fixup outputs (env and DLL override fragments) into the final environment/overrides in wine_tkg runner.
• Implemented a working Rhai fixup subsystem in src/launch/fixups/mod.rs with FixupContext/FixupResult, seed scripts (3–5 examples), ResolveGameFixupsStage in the pipeline, and wiring to PipelineContext/Runner LaunchContext so results are applied before final command construction.
• Instrumented launch verification with protonfixes_routed: bool and rhai_fixup_applied: Option<String> in LaunchVerification and included these fields in the concise launch summary metadata; added a small UI validation warning for Steam Runtime Runner pointing at unknown paths.
• Added rhai = { version = "1", features = ["sync"] } to Cargo.toml and included unit tests for the fixup loader (including malformed-script handling).

Testing

• Ran the full test suite with cargo test -q, and all new and existing unit tests passed (tests exercised classify_runner, Rhai fixup loader including malformed-script behavior, and runner routing logic).
• Attempted cargo build --release -q in this CI-like environment but the build process timed out before completion; no compiler errors were emitted during the work that produced these changes.
• Added/updated unit tests: runner classification tests, fixup seed-script execution test, malformed-script error test, and ensured existing infra/runner tests were updated and continue to pass under cargo test.

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Codex Task