fix: incremental rootfs builds so stale archives are never deployed

The deploy failed with "grub-install: command not found" in the chroot —
the installed rootfs archive was built BEFORE the BOOT/grub injection, so
it had no bootloader at all. build-iso.sh / build-image.sh only rebuilt
the rootfs when the archive was missing, never when sources changed, so
development fixes were silently absent from deployed images.

- build-rootfs.sh: incremental staleness check — skips a rebuild only when
  no input (scripts/, btrfs/, overlays/, editions/<e>/, versions.mk,
  toolchain debs) is newer than the artifact; FORCE=1 rebuilds anyway.
- build-iso.sh / build-image.sh: always delegate freshness to
  build-rootfs.sh instead of gating on file existence.
- deploy-disk.sh: defensive check that grub-install exists in the deployed
  rootfs, with a clear "stale archive — rebuild" message instead of a bare
  "command not found".
- docs/building.md: incremental-build note (FORCE=1 / make clean).

scripts/build-rootfs.sh

@@ -42,6 +42,27 @@ ROOTFS="$ROOTFS_DIR/$EDITION"
 ARTIFACT="$ARTIFACT_DIR/arcline-$EDITION-$VERSION-$ARCH.tar.xz"
 
 mkdir -p "$ROOTFS_DIR" "$LOG_DIR" "$ARTIFACT_DIR"
+
+# ── staleness check ─────────────────────────────────────────────────────────
+# Incremental builds: skip rebuilding when no input changed since the last
+# artifact. This is what makes `make iso-<edition>` fast when nothing changed,
+# AND it catches stale archives during development (the bug class that shipped
+# a rootfs with no grub-install). Force with FORCE=1 or `make clean`.
+newer_input() { # <artifact> → 0 if any input is newer than the artifact
+  local art="$1"
+  [[ -f "$art" ]] || return 0
+  find "$ROOT/scripts" "$ROOT/btrfs" "$ROOT/overlays" "$EDIR" \
+       -type f -newer "$art" -print -quit 2>/dev/null | grep -q . && return 0
+  [[ -d "$DEB_DIR" ]] && find "$DEB_DIR" -name '*.deb' -newer "$art" -print -quit 2>/dev/null | grep -q . && return 0
+  [[ "$ROOT/versions.mk" -nt "$art" ]] && return 0
+  return 1
+}
+
+if [[ "${FORCE:-0}" != "1" && -d "$ROOTFS" && -f "$ARTIFACT" ]] && ! newer_input "$ARTIFACT"; then
+  log "rootfs '$EDITION' is up to date — skipping rebuild ($ARTIFACT)"
+  log "  (FORCE=1 rebuilds anyway)"
+  exit 0
+fi
 rm -rf "$ROOTFS" "$ARTIFACT"
 
 log "═══ building Arcline $EDITION rootfs ($DISTRO_NAME $VERSION) ═══"