deb,cli,test,docs: record and show dep-install and build durations

Every binary build measures its dependency installation (apt-get
update, the essential and injected packages, each build-dependency
pass) and its package build (debian/rules build plus binary) into a
timings accumulator; the record manifest stores both as an optional
`timings` field when the run ends, whatever the outcome — a failed
build keeps its partial durations, and a run that died before any
timed command records nothing.

`pkh deb list` shows the two durations as DEPS and BUILD columns,
formatted like the age ("45s", "2m41s", "1h02m"); records predating
the field show dashes. The field is serde-optional, so the manifest
schema stays at 1 and existing records remain discoverable and
forkable.

src/deb/mod.rs

@@ -458,6 +458,10 @@ async fn build_binary_package_impl(
         None => base_ctx.clone(),
     };
 
+    // The measured durations of the build's work phases, recorded on the
+    // record when the run ends — success or failure alike.
+    let mut timings = session::BuildTimings::default();
+
     let result = async {
         // Prepare build directory: the stable /tmp/pkh-build for session
         // builds (see EphemeralContextGuard::build_root — the overlay
@@ -571,6 +575,7 @@ async fn build_binary_package_impl(
                     session.as_ref(),
                     build_resume,
                     ccache,
+                    &mut timings,
                 )
                 .await?
             }
@@ -617,6 +622,14 @@ async fn build_binary_package_impl(
     {
         s.set_outcome(session::OUTCOME_FAILED);
     }
+    // The measured durations land in the record whatever the outcome: a
+    // failed build's partial timings are data too. Nothing measured (the
+    // run ended before any timed command) records nothing.
+    if let Some(s) = &session
+        && !timings.is_empty()
+    {
+        s.set_timings(timings);
+    }
 
     result
 }