feat: complete native execution and Steam discovery

docs/phase-1/evidence-completion-2026-07-18.md

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+# Phase 1 completion evidence
+
+- Date: 2026-07-18
+- Requirement IDs: R-009, R-012, R-016, R-018, R-024, R-029
+- Phase/gate: phase 1 / provider-independent contract completion
+- Scope: provider/runtime capability interfaces, typed dry-run launch plans, lifecycle
+  confidence policy, structured versioned errors, fixed-precedence versioned TOML with
+  provenance, deterministic mock scenarios, and versioned CLI machine output
+- Exclusions: real providers, process execution, daemon or IPC, discovery cache, GUI,
+  session supervision, installation management, public stability, and final naming
+- Commands: `make verify`; `make audit`; `cargo test --workspace --all-targets --locked`;
+  human and JSON `kiln doctor` smoke paths; structured CLI error smoke paths
+- Expected result: all required Phase 1 mock outcomes and hostile inputs are covered;
+  schemas reject unknown versions and fields; launch arguments never require a shell;
+  lifecycle effects degrade conservatively; configuration precedence is deterministic
+- Observed result: 40 unit, contract, adversarial, persistence, concurrency, mock, and
+  isolated CLI integration tests passed; formatting, compilation, Clippy with warnings
+  denied, dependency policy, source inventory, schema reports, and smoke checks passed
+- Result: pass