feat: page titles must be valid, unique file names on Windows and macOS - new/rename/move refuse, lint reports Unportable Titles, new never overwrites (#155)

Files changed:
- CHANGES.md
- README.md
- VERSION
- instructions/page-lifecycle.md
- instructions/wiki-lint/SKILL.md
- kb/CONTRACT.md
- kb/CONVENTIONS.md
- kb/CONVENTIONS.md.template
- tools/CONTRACT.md
- tools/chemenu/commands/_util.py
- tools/chemenu/commands/lint.py
- tools/chemenu/commands/new_page.py
- tools/chemenu/commands/page_ops.py
- tools/chemenu/lint_core.py
- tools/chemenu/tests/test_lint.py
- tools/chemenu/tests/test_new_page.py
- tools/chemenu/tests/test_page_ops.py
- tools/chemenu/tests/test_titles.py
- tools/chemenu/titles.py

kb/CONVENTIONS.md

@@ -96,8 +96,9 @@ What to name a thing: projects use their repository or common name; systems a de
 name; tools the tool's own name; technologies their standard spelling and capitalization;
 people a full name or common handle.
 
-The one naming fact that is *not* a choice, and therefore lives in the contract: the filename
-stem is the page title, and `[[wikilinks]]` must match it exactly.
+The naming facts that are *not* a choice, and therefore live in the contract: the filename
+stem is the page title, `[[wikilinks]]` must match it exactly, and the title must be a valid,
+unique file name on every platform (`kb/CONTRACT.md` § Titles are identifiers).
 
 ## Tone