blockdoc — one block per file

Long nested files break under surgical agent edits: mismatched tags, wrong nesting depth, off-by-one replacements. The fix is a practice change, not better editing: one block per part file, append-only assembly, validate at build. The parts directory is the source of truth; the assembled file is a build artifact you never hand-edit.

Tool

scripts/blockdoc.py (Python stdlib only):

python3 scripts/blockdoc.py split <file.html> <parts-dir>   adopt an existing long file
python3 scripts/blockdoc.py build <parts-dir> [outfile]     assemble + tag-balance validate (gate)
python3 scripts/blockdoc.py check <file.html>               validate one file in place
python3 scripts/blockdoc.py selftest                        built-in checks