feat(peer): prototype streamed installs

Add a streamed-install prototype that can receive archive-derived install bytes
straight into local/ without first storing the peer-owned root archive payload.
This is intended for low-disk clients that want to install a game but opt out of
becoming a downloadable peer source for that game.

The protocol gains a current-version-only StreamInstall request and framed
StreamInstallFrame responses. The peer core owns the generic transport,
transaction, path validation, size checks, CRC32 verification, and lifecycle
state. The archive-specific work is hidden behind StreamInstallProvider so the
prototype can use unrar while the final implementation can swap in a better
provider without rewriting the peer command path.

The receiver writes into .local.installing and only promotes to local/ after the
full stream verifies. It deliberately does not write the root version.ini or
archive files, so the settled local state is installed=true, downloaded=false,
and availability=LocalOnly. That preserves the existing rule that local/ is not
served to peers and makes streamed receivers non-sources by construction.

The CLI is the only caller for now. It exposes stream-install and provides the
prototype unrar implementation with unrar lt for entry metadata and unrar p for
file bytes. This is simple and good enough to prove non-solid archive streaming,
but it is not the production provider shape for solid archives because per-file
unrar p would repeatedly decompress prefixes. The Tauri app explicitly passes
stream_install_provider: None, so the GUI behavior stays unchanged until a real
product path is designed.

Document the production-readiness work in NEXT_STEPS.md. The main follow-up is
to make the provider abstraction final-ish and replace the per-file CLI unrar
provider with a one-pass archive provider, then wire a deliberate GUI low-disk
mode, retry semantics, and broader failure scenarios.

Test Plan:
- just fmt
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just test
- python3 crates/lanspread-peer-cli/scripts/run_extended_scenarios.py \
  S39 S40 --build-image
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just clippy
- git diff --check
- git diff --cached --check

Follow-up: NEXT_STEPS.md

crates/lanspread-peer/src/install/mod.rs

@@ -4,5 +4,13 @@ mod transaction;
 pub mod unpack;
 
 pub use remove::remove_downloaded;
-pub use transaction::{install, recover_on_startup, uninstall, update};
+pub(crate) use transaction::root_eti_archives;
+pub use transaction::{
+    StreamedInstallTransaction,
+    begin_streamed_install,
+    install,
+    recover_on_startup,
+    uninstall,
+    update,
+};
 pub use unpack::{UnpackFuture, Unpacker};

crates/lanspread-peer/src/install/transaction.rs

@@ -33,6 +33,103 @@ struct InstallFsState {
     backup: FsEntryState,
 }
 
+pub struct StreamedInstallTransaction {
+    game_root: PathBuf,
+    state_dir: PathBuf,
+    id: String,
+    staging: PathBuf,
+    eti_version: Option<String>,
+}
+
+impl StreamedInstallTransaction {
+    #[must_use]
+    pub fn staging_dir(&self) -> &Path {
+        &self.staging
+    }
+
+    pub async fn commit(self) -> eyre::Result<()> {
+        let local = local_dir(&self.game_root);
+        let result = async {
+            tokio::fs::rename(&self.staging, &local)
+                .await
+                .wrap_err_with(|| format!("failed to promote streamed install for {}", self.id))?;
+            reset_launch_settings_marker(&self.state_dir, &self.id).await?;
+            write_intent(
+                &self.state_dir,
+                &self.id,
+                &InstallIntent::none(&self.id, self.eti_version.clone()),
+            )
+            .await
+        }
+        .await;
+
+        if result.is_err() {
+            if let Err(cleanup_err) = remove_dir_all_if_exists(&self.staging).await {
+                log::warn!(
+                    "Failed to clean streamed install staging {}: {cleanup_err}",
+                    self.staging.display()
+                );
+            }
+            let _ = write_intent(
+                &self.state_dir,
+                &self.id,
+                &InstallIntent::none(&self.id, self.eti_version.clone()),
+            )
+            .await;
+        }
+
+        result
+    }
+
+    pub async fn rollback(self) -> eyre::Result<()> {
+        let staging_result = remove_dir_all_if_exists(&self.staging).await;
+        let intent_result = write_intent(
+            &self.state_dir,
+            &self.id,
+            &InstallIntent::none(&self.id, self.eti_version.clone()),
+        )
+        .await;
+
+        staging_result?;
+        intent_result
+    }
+}
+
+pub async fn begin_streamed_install(
+    game_root: &Path,
+    state_dir: &Path,
+    id: &str,
+) -> eyre::Result<StreamedInstallTransaction> {
+    if path_is_dir(&local_dir(game_root)).await {
+        eyre::bail!("game {id} is already installed");
+    }
+
+    tokio::fs::create_dir_all(game_root).await?;
+    let eti_version = read_downloaded_version(game_root).await;
+    write_intent(
+        state_dir,
+        id,
+        &InstallIntent::new(id, InstallIntentState::Installing, eti_version.clone()),
+    )
+    .await?;
+
+    let staging = installing_dir(game_root);
+    if let Err(err) = prepare_owned_empty_dir(&staging).await {
+        let _ = write_intent(state_dir, id, &InstallIntent::none(id, eti_version)).await;
+        return Err(err);
+    }
+
+    let staging = tokio::fs::canonicalize(&staging).await.unwrap_or(staging);
+
+    Ok(StreamedInstallTransaction {
+        game_root: game_root.to_path_buf(),
+        state_dir: state_dir.to_path_buf(),
+        id: id.to_string(),
+        staging,
+        eti_version,
+    })
+}
+
 pub async fn install(
     game_root: &Path,
     state_dir: &Path,
@@ -258,7 +355,7 @@ async fn unpack_archives(
     Ok(())
 }
 
-async fn root_eti_archives(game_root: &Path) -> eyre::Result<Vec<PathBuf>> {
+pub(crate) async fn root_eti_archives(game_root: &Path) -> eyre::Result<Vec<PathBuf>> {
     let mut entries = tokio::fs::read_dir(game_root).await?;
     let mut archives = Vec::new();
     while let Some(entry) = entries.next_entry().await? {