feat(progress): report uncounted status through the sink

A run blocked on the cache-entry flock was silent: the wait went to
the log sink, which renders nowhere by default, so the CLI read as
a hang. The Progress trait gains message() - a status line outside
the counted-operation protocol, defaulting to a no-op so existing
sinks keep compiling - and the flock wait reports what it is
waiting for. The CLI renders messages on the active bar and spins
one up when no operation is active.

crates/ecrou/src/cache.rs

@@ -94,9 +94,11 @@ pub(crate) struct EntryLock {
 }
 
 impl EntryLock {
-    /// Block until the entry is free. `no_cache` callers skip the
-    /// lock entirely: an explicit fresh download never waits.
-    pub(crate) fn acquire(entry: &Path) -> anyhow::Result<Self> {
+    /// Block until the entry is free, telling the sink what the run
+    /// is waiting on (a silent block reads as a hang). `no_cache`
+    /// callers skip the lock entirely: an explicit fresh download
+    /// never waits.
+    pub(crate) fn acquire(entry: &Path, progress: &dyn Progress) -> anyhow::Result<Self> {
         use std::os::fd::AsRawFd;
 
         let lock_path = entry.with_extension("lock");
@@ -120,6 +122,10 @@ impl EntryLock {
                 "waiting for another ecrou run to finish preparing {}",
                 entry.display()
             );
+            progress.message(&format!(
+                "Waiting for another ecrou run to finish preparing {}",
+                entry.display()
+            ));
             let held = unsafe { libc::flock(file.as_raw_fd(), libc::LOCK_EX) };
             if held != 0 {
                 return Err(anyhow::Error::from(std::io::Error::last_os_error())

crates/ecrou/src/progress.rs

@@ -19,7 +19,9 @@ pub enum Unit {
 ///
 /// Operations are sequential: `begin` starts the next operation (any
 /// previous one is implicitly over), `advance` counts completed units,
-/// `finish` marks its end.  Sinks must be callable from any thread.
+/// `finish` marks its end.  `message` reports uncounted status outside
+/// that protocol — a wait, a decision, a retry — when there is nothing
+/// to count.  Sinks must be callable from any thread.
 pub trait Progress: Send + Sync {
     /// An operation labelled `label` starts, counting `unit`s; `total`
     /// is the unit count when known upfront.
@@ -30,6 +32,11 @@ pub trait Progress: Send + Sync {
 
     /// The current operation ended.
     fn finish(&self);
+
+    /// A status line outside the counted-operation protocol. The
+    /// default discards it; rendering sinks surface it as a spinner
+    /// message, starting one if no operation is active.
+    fn message(&self, _text: &str) {}
 }
 
 /// A [`Progress`] sink that discards every event.

crates/ecrou/src/rootfs.rs

@@ -88,7 +88,8 @@ impl RootfsCache {
                 // One writer per entry: a concurrent run preparing the
                 // same image waits here instead of downloading a second
                 // copy the atomic rename would throw away.
-                let _entry_lock = EntryLock::acquire(&cache_path).map_err(Error::Other)?;
+                let _entry_lock =
+                    EntryLock::acquire(&cache_path, progress).map_err(Error::Other)?;
                 // Re-derive the freshness answer under the lock: a
                 // competing run may have filled or refreshed the entry
                 // while we waited.