fix(core): prune old installed app snapshots

[peters]

Summary

• wire installed-version retention through the core update manager, FFI, and .NET wrapper
• prune stale app-* snapshots after successful updates while keeping one rollback snapshot by default
• add regression coverage for retained and zero-retention update flows

Behavior Impact

• successful updates now stop accumulating old app-* directories indefinitely
• SurgeUpdateManager.ReleaseRetentionLimit now affects native update behavior instead of being ignored
• default behavior retains app/ plus one previous installed version snapshot

Test Evidence

• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic
• dotnet format dotnet/Surge.slnx --verify-no-changes
• dotnet test dotnet/Surge.slnx --configuration Release

Migration Notes

• none; existing stale app-* directories on already-updated installs will be pruned by the next successful update with this build