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/startup.rs

@@ -23,6 +23,7 @@ use crate::{
     PeerCommand,
     PeerEvent,
     PeerRuntimeComponent,
+    StreamInstallProvider,
     Unpacker,
     context::Ctx,
     events,
@@ -87,6 +88,7 @@ pub(crate) fn spawn_peer_runtime(
     unpacker: Arc<dyn Unpacker>,
     catalog: Arc<RwLock<GameCatalog>>,
     active_outbound_transfers: crate::context::OutboundTransfers,
+    stream_install_provider: Arc<dyn StreamInstallProvider>,
 ) -> PeerRuntimeHandle {
     let shutdown = CancellationToken::new();
     let task_tracker = TaskTracker::new();
@@ -107,6 +109,7 @@ pub(crate) fn spawn_peer_runtime(
             runtime_tracker.clone(),
             catalog,
             active_outbound_transfers,
+            stream_install_provider,
         )
         .await
         {