Validate manifest installdir and recover install path via steam_appid probe

[weter11]

Motivation

• Manifests can contain stale installdir names (e.g. App <id>) that no longer match the actual folder name, causing incorrect install path resolution and preflight failures.
• The scan should prefer a verified install directory and recover from stale manifests by probing for app-id markers in installed folders.

Description

• Update parse_app_manifest_info in src/library.rs to validate the derived steamapps/common/<installdir> before using it as the canonical install path. The validation checks for a matching steam_appid.txt or at least the presence of files in the folder.
• Add is_valid_install_path, probe_install_dir_by_appid, and rewrite_manifest_installdir helpers to src/library.rs to perform the validation, probe steamapps/common/*/steam_appid.txt for the app ID, and optionally rewrite the manifest installdir when a different folder name is recovered.
• Fall back to probing steamapps/common/*/steam_appid.txt when the manifest-derived path is invalid, and use the recovered folder as the canonical install path (mirrors SteamClient::probe_install_dir_by_appid strategy in src/steam_client.rs).
• Add regression test tests/library_scan_fallback.rs which simulates a manifest with installdir = "App 2410180" while the actual folder is Portal Prelude RTX containing steam_appid.txt = 2410180, asserting the scan resolves to the real folder and that the manifest is rewritten.

Testing

• Ran code formatting with cargo fmt --all which completed successfully.
• Ran the targeted tests with cargo test --test library_scan_fallback --test path_resolution and both tests completed successfully (all ran and passed).

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Codex Task