deb: do not retain staged trees over the retention size cap

The per-target retention kept the newest five staged trees however
large they are: five carried kernel trees are three-digit GiB, most of
it compiled objects whose resume value does not scale with their size.

A staged tree larger than the retention cap (a user setting, see
below) is now dropped at the end of its own build — the record
degrades to metadata-only and the next build re-stages from scratch —
and the retention sweep also degrades older oversized trees, covering
records kept by older pkh versions and interrupted builds. The cap of
0 disables the size rule; the count cap is unchanged.

The cap is the first setting of a user configuration file,
~/.config/pkh/config.json (the directory already holds contexts.json):
missing file and missing keys mean defaults, a corrupt file is backed
up and the defaults apply, and PKH_STAGED_MAXSIZE overrides the file.
The default is 20G, the scale where a carried kernel tree stops
earning its disk.

Every retained tree's size is measured once at retention (the same
concurrent, mount-crossing-safe walker the prune report uses) and
stored in the record manifest, so sweeps reuse it instead of re-walking
the trees on every build.

src/lib.rs

@@ -9,6 +9,9 @@ pub mod apt;
 pub mod build;
 /// Parse or edit a Debian changelog of a source package
 pub mod changelog;
+/// User settings: the optional `config.json` in the pkh configuration
+/// directory, its defaults and the environment overrides
+pub mod config;
 /// Embedding convention for static reference data (`data/*.yml`), applied
 /// by each owning module via the `embed_data!` macro
 pub(crate) mod data;