deb: record per-phase build timings, shown by pkh deb profile

Time every Phase the build flow announces through enter_phase — the
chroot bootstrap (tarball download, extraction, finalization), the
apt phases, patch application, the build and binary steps and the
artifact retrieval — and land the ordered profile in the record's
manifest of every run, failed builds included: their profile ends
at the phase the error propagated out of.

Timing rides the enter_phase choke point instead of new measurement
sites, so a phase added later profiles itself. The manifest field
is additive (serde default, skipped when absent), so older manifests
keep loading without a schema bump. The coarse timings stay the
source of the deb list columns: they measure the apt transactions
themselves, where the profile measures whole phases.

pkh deb profile renders a record's profile through the same
read-only record discovery as pkh deb log — the newest record of
the tree, or an id prefix as shown by pkh deb list — so
record_for_log becomes the neutral find_record. Phase keys are
storage names, distinct from the display labels; Phase::from_key
turns them back and a test keeps both in sync.

README.md

@@ -174,6 +174,11 @@ pkh deb list            # the records of this tree: id, outcome,
                         # dep-install and build times, age
 pkh deb log [-f] [<id>] # print a record's build log; -f follows a
                         # live build
+pkh deb profile [<id>]  # a record's per-phase timing profile: how
+                        # long the chroot bootstrap, each apt phase,
+                        # patch application, the build and binary
+                        # steps and the artifact retrieval took, with
+                        # their shares of the total
 pkh deb --no-ccache     # build without the shared compile cache
 ```