feat: complete Steam execution

docs/phase-4/evidence-2026-07-20.md

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+# Phase 4 Steam execution evidence
+
+- Date: 2026-07-20
+- Requirement IDs: R-005, R-016, R-018, R-024
+- Phase/gate: phase 4 / Steam execution
+- Scope: typed native and Flatpak Steam launch plans, configuration provenance,
+  argument-safe client submission, private logs, structured errors, and representative
+  native plus Proton execution
+- Exclusions: direct executable launch, provider-client ownership, strong game-lifetime
+  tracking, automatic Steam shutdown, and minimal-session behavior
+- Host: Linux 7.1.4-arch1-1; NVIDIA GeForce RTX 5070 using the `nvidia` driver;
+  Steam 1.0.0.87-1
+- Commands: `kiln launch steam:105600 --dry-run --json`; `kiln launch steam:105600
+  --json`; `kiln launch steam:12810 --dry-run --json`; `kiln launch steam:12810
+  --json`; `make check`
+- Expected result: Kiln emits a typed `steam -silent -applaunch <appid>` plan, submits
+  it without shell interpolation, returns promptly with unknown lifecycle confidence,
+  launches one native and one Proton title through Steam, and reports missing clients
+  as structured failures
+- Observed result: Terraria App ID 105600 loaded through Steam Linux Runtime and its
+  native `Terraria.bin.x86_64`; Overlord II App ID 12810 loaded through
+  `GE-Proton11-1` and `Overlord2.exe`; both returned cleanly after user exit
+- Safety and cleanup: Steam was stopped before the temporary compatibility change and
+  final restore. The original `config.vdf` was restored byte-for-byte with SHA-256
+  `9251c6dbf08602857399916808208bd58dd415b5d5b000acc01f1c9044d89c37`, mode and
+  ownership unchanged, and Steam restarted
+- Automated result: 61 tests passed with the complete local gate
+- Result: pass