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/main.rs

@@ -383,7 +383,7 @@ fn main() {
                 .subcommand(
                     Command::new("list")
                         .about("List the build records recorded from this tree")
-                        .long_about("List the build records recorded from this tree (plain runs too, as metadata records), with their outcome, version, fork lineage, whether their staged tree is kept, and the id to pass to 'pkh deb --incremental <id>'.\nRead-only: never builds or removes anything (at most five staged trees per target are kept, 'pkh prune' removes records older than a week)."),
+                        .long_about("List the build records recorded from this tree (plain runs too, as metadata records), with their outcome, version, fork lineage, whether their staged tree is kept, how long the dependency installation and the package build took, and the id to pass to 'pkh deb --incremental <id>'.\nRead-only: never builds or removes anything (at most five staged trees per target are kept, 'pkh prune' removes records older than a week)."),
                 )
                 .subcommand(
                     Command::new("log")