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/page-lifecycle.md

@@ -14,6 +14,17 @@ frontmatter reference arrays (`related:`, `sources:`, `entities:`, `concepts:`).
 hand.** Each of the commands below rewrites all three places at once; hand-editing rewrites
 one and leaves the others pointing at nothing.
 
+<!-- wikitool:toc -->
+## Contents
+
+- [Rename](#rename)
+- [Delete](#delete)
+- [Move](#move)
+- [Drop a single reference](#drop-a-single-reference)
+- [Afterwards](#afterwards)
+- [Scope](#scope)
+<!-- /wikitool:toc -->
+
 ## Rename
 
 ```bash
@@ -25,6 +36,11 @@ Repoints body wikilinks (aliases and anchors preserved), a citation id derived f
 title (both its Footnotes definition and every `[^cite-id]` reference to it), the page's own
 H1, and every frontmatter reference array the type declares in `page_ref_fields:`.
 
+`--to` has to be a valid, unique file name on every platform, and `--dry-run` refuses it the
+same way the real run does. The rule is in `kb/CONTRACT.md` § Titles are identifiers. Only `--to`
+is checked, so this is also the fix for `lint`'s **Unportable Titles** finding: rename the page
+away from the title that breaks the rule. A change of case alone (`Foo` to `FOO`) is allowed.
+
 **If `--from` is not a page but is referenced**, rename instead repoints those references onto
 the existing `--to` page and moves nothing. That is the fix for a reference spelled
 `act_runner` when the page is `Act Runner`.
@@ -62,9 +78,10 @@ reference anywhere in the wiki needs updating.
 to do. `wikitool lint`'s **Misplaced Pages** finding is the advisory this fixes - it is not a
 hard error, so an unreconciled corpus is not a broken one, only one `move` would tidy.
 
-A destination that already holds a file with the page's name is refused, not silently
-overwritten - that only happens on a pre-existing duplicate-title collision, which `lint`'s
-**Duplicate Titles** finding reports separately.
+A destination that already holds a file with the page's name - or one that differs from it only
+in case or Unicode normalization - is refused, not silently overwritten. That only happens on a
+pre-existing duplicate-title collision, which `lint`'s **Duplicate Titles** and **Unportable
+Titles** findings report separately.
 
 ## Drop a single reference