Improve --generate performance with incremental builds (62s -> 2s)

Add mtime-based skip logic to avoid regenerating unchanged files:
- generate::fromgmi skips .gmi files where all outputs are newer
- template::_generate_file skips templates when output is fresh
- Diff-before-overwrite in templates, gemfeed, and notes indexes
  to preserve mtimes and prevent cascading cache invalidation
- Global dependency check (.lastgen sentinel) for header/footer/CSS
- Job throttling via wait -n capped at nproc cores
- Add --force flag and FORCE_REBUILD env var to bypass skip logic
- Fix misleading atom feed "empty cache" log message

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/lib/gemfeed.source.sh b/lib/gemfeed.source.sh
index f069ef5..c405083 100644
--- a/lib/gemfeed.source.sh
+++ b/lib/gemfeed.source.sh
@@ -19,7 +19,12 @@ gemfeed::updatemainindex () {
     $SED -E -n '/^=> / { s| ./| ./gemfeed/|; p; }' \
         "$gemfeed_dir/index.gmi" >> "$index_gmi.tmp"
 
-    mv "$index_gmi.tmp" "$index_gmi"
+    # Only overwrite if content changed, preserving mtime for template skip logic
+    if [[ -f "$index_gmi" ]] && diff -q "$index_gmi.tmp" "$index_gmi" >/dev/null 2>&1; then
+        rm "$index_gmi.tmp"
+    else
+        mv "$index_gmi.tmp" "$index_gmi"
+    fi
 }
 
 gemfeed::_get_word_count () {
@@ -59,7 +64,12 @@ GEMFEED
             "$gemfeed_dir/index.gmi.tmp"
     done < <(gemfeed::get_posts)
 
-    mv "$gemfeed_dir/index.gmi.tmp" "$gemfeed_dir/index.gmi"
+    # Only overwrite if content changed, preserving mtime for template skip logic
+    if [[ -f "$gemfeed_dir/index.gmi" ]] && diff -q "$gemfeed_dir/index.gmi.tmp" "$gemfeed_dir/index.gmi" >/dev/null 2>&1; then
+        rm "$gemfeed_dir/index.gmi.tmp"
+    else
+        mv "$gemfeed_dir/index.gmi.tmp" "$gemfeed_dir/index.gmi"
+    fi
 
     gemfeed::updatemainindex
 }