feat(apps): snap backend — plan operations, execution, detection, facade wiring

crates/toride-apps/README.md

@@ -9,9 +9,13 @@ manifest.
   manager argv from a registry `App` plus a host `Target` — no I/O,
   fixture-testable.
 - The `Backend` trait with executed implementations for Homebrew,
-  Flatpak, distro managers (apt, dnf, pacman, apk), npm, cargo, pipx,
-  uv, mise-delegated tools, and direct downloads — install, uninstall,
-  update, status, version queries, and pinning.
+  Flatpak, snap, distro managers (apt, dnf, pacman, apk), npm, cargo,
+  pipx, uv, mise-delegated tools, and direct downloads: install,
+  uninstall, update, status, and version queries. Pinning is
+  Homebrew-only; the trait default refuses it for every other backend.
+  Snap has no registry `InstallMethod` routing to it yet: its
+  install/remove/refresh plans are built at the operation seam, and
+  records carry the snap name for uninstall and update replay.
 - The `Apps` facade: ensure-installed (optionally at a version) and
   uninstall, each with a per-item batch verb
   (`ensure_installed_many` / `uninstall_many`), update with dry-run,

crates/toride-apps/src/backend.rs

@@ -46,6 +46,8 @@ pub enum BackendId {
     Homebrew,
     /// Linux Flatpak — remotes and refs (`flatpak`).
     Flatpak,
+    /// Linux Snap packages, through snapd (`snap`).
+    Snap,
     /// A distro's native package manager, selected by family (`apt`, `dnf`,
     /// `pacman`, `apk`).
     Distro(DistroFamily),
@@ -72,6 +74,7 @@ impl std::fmt::Display for BackendId {
         match self {
             Self::Homebrew => f.write_str("homebrew"),
             Self::Flatpak => f.write_str("flatpak"),
+            Self::Snap => f.write_str("snap"),
             // Families render lowercase (`distro-debian`); the wildcard keeps
             // future families displayable without touching this crate.
             Self::Distro(family) => {

crates/toride-apps/src/backends/mod.rs

@@ -16,6 +16,7 @@ pub mod homebrew;
 pub mod mise;
 pub mod npm;
 pub mod pipx;
+pub mod snap;
 pub mod uv;
 
 pub use cargo::CargoBackend;
@@ -28,4 +29,5 @@ pub use homebrew::HomebrewBackend;
 pub use mise::MiseBackend;
 pub use npm::NpmBackend;
 pub use pipx::PipxBackend;
+pub use snap::SnapBackend;
 pub use uv::UvBackend;