deb: add opt-in --incremental flag for single-flag iterate loops

Iterating on a package currently needs two flags (--resume --keep):
without --keep a successful build consumes the session, and without
--resume nothing is reused, so incremental iteration never "just
works" opt-in.

--incremental is the single-flag form: with no --resume it adopts the
newest usable session of this tree (like `pkh deb --resume`), and it
always keeps the session on success, so the loop (edit, `pkh deb
--incremental`, edit, ...) reuses the chroot, build deps and objects
and only recompiles what changed. With an explicit `--resume <id>` it
only adds keep.

A plain `pkh deb` is unchanged: fresh session, consumed on success.

src/main.rs

@@ -376,6 +376,8 @@ fn main() {
                 )
                 .arg(arg!(--keep "Keep the build session after a successful build (for a later --resume)")
                     .long_help("Keep the build session after a successful build, so a later 'pkh deb --resume' can iterate on it (e.g. after editing sources).\nWithout it, a successful build consumes the session (failures and interrupts always keep it)."))
+                .arg(arg!(--incremental "Reuse a previous build session of this target (like --resume) and keep the session even on success; the next --incremental build reuses the chroot, build deps and objects")
+                    .required(false))
                 .arg(arg!(--verbose "Show raw tool output instead of the live build view").required(false)
                     .long_help("Show raw tool output instead of the live build view.\nAlso implied by RUST_LOG=debug for pkh's own logs."))
                 .subcommand(
@@ -937,14 +939,18 @@ fn main() {
                 })
             });
 
-            // Session handling: --resume [<id>] and --keep (see the deb
-            // subcommand help).
+            // Session handling: --resume [<id>], --incremental and --keep
+            // (see the deb subcommand help). --incremental adopts the newest
+            // session when no explicit --resume ID is given, and always adds
+            // keep so the session survives success.
+            let incremental = sub_matches.get_flag("incremental");
             let resume = match sub_matches.get_one::<String>("resume").map(String::as_str) {
                 Some("") => pkh::deb::SessionResume::Latest,
                 Some(id) => pkh::deb::SessionResume::Id(id.to_string()),
+                None if incremental => pkh::deb::SessionResume::Latest,
                 None => pkh::deb::SessionResume::Fresh,
             };
-            let keep_session = sub_matches.get_flag("keep");
+            let keep_session = sub_matches.get_flag("keep") || incremental;
 
             // Live build view, unless --verbose (DebUi additionally disables
             // itself when stdout is not a terminal)