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

instructions/wiki-lint/SKILL.md

@@ -42,7 +42,8 @@ mechanical half looks exactly like a complete one.
    No flags: prints the sections that found something, writes the full report to
    `reports/Lint Report <YYYY-MM-DD>.md`, and names that path. This deterministically finds
    unreadable frontmatter, broken wikilinks, dangling frontmatter references, orphan pages,
-   catalog drift, missing fields, duplicate titles, filename/title mismatches, broken
+   catalog drift, missing fields, duplicate titles, titles that are not valid, unique file
+   names on Windows and macOS, filename/title mismatches, broken
    `raw_files:` references, raw files claimed by more than one source page, invalid type paths,
    schema failures, citation/frontmatter drift, and edges whose label is missing, not authorised
    by the source collection, or redundant beside a specific label on the reverse direction.