feat(release): changes/ fragment system + generator, .public-surface snapshot + CI migration gate, deprecation-warning + anchor harnesses, Trusted Publishing, relicense CHANGELOG entry

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+Change fragments
+================
+
+Every breaking change to Edify's public surface lands with a **change fragment**
+in this directory. Fragments are the single source of truth for the CHANGELOG and
+the upgrade guide: the release generator (``tools/changes.py``) concatenates them
+into both outputs, then deletes them.
+
+There is no external changelog tooling — the format is a small stdlib-parseable
+INI, and the generator is a single stdlib script.
+
+File format
+-----------
+
+One file per breaking change, named ``<fragment-id>.rst`` where ``<fragment-id>``
+sorts into the order the fragment should appear in the release notes (for
+example ``0010-char-class-escape.rst``). The file is an INI document with a single
+``[change]`` section:
+
+.. code-block:: ini
+
+    [change]
+    anchor = char-class-escape
+    heading = Character-class escaping is the minimal correct form
+    before = any_of_chars('#?!@$%^&*-')  ->  [\#\?!@$%\^\&\*\-]
+    after = any_of_chars('#?!@$%^&*-')  ->  [#?!@$%^&*-]
+    context = Only backslash, closing bracket, first-position caret, and interior
+        dash are escaped inside a character class; every other metacharacter is a
+        literal. Match behavior is unchanged.
+
+Fields
+------
+
+``anchor`` (required)
+    The frozen ``.. _<anchor>:`` label the upgrade-guide section carries. The
+    deprecation-warning URL for this change ends in ``#<anchor>``.
+
+``heading`` (required)
+    The one-line human title of the change.
+
+``before`` / ``after`` (optional)
+    A before/after pair. Include both or neither. Rendered as a literal block when
+    present.
+
+``context`` (required)
+    One or two sentences explaining what changed and what the caller should do.
+    May span multiple indented lines.
+
+Release flow
+------------
+
+On release, ``tools/changes.py`` slots each fragment into the matching section of
+``CHANGELOG.rst`` and the active upgrade guide in fragment-id order, then removes
+the fragment files. Adding a fragment is the only step a contributor takes; the
+generator does the rest.