Note compat Router shim breaking changes in changelog

PR #758 review surfaced two compat-shim behaviour changes that are
breaking versus the previous `@fedify/fedify` `Router` and were not
called out in the changelog: `Router#route()` now calls `assertPath()`
and throws `RouterError` for non-path inputs (previously returned
`null`), and `trailingSlashInsensitive` is fixed at construction time
(post-construction assignments create an inert own property on the
wrapper). Both are intentional — the shim is deprecated and its
surface is intentionally constrained — but they need a migration
note so consumers can either pass options at the constructor or move
to `Router` from `@fedify/uri-template`.

#758 (comment)
#758 (comment)

Assisted-by: Claude Code:claude-opus-4-7

Author: 2chanhaeng

CHANGES.md

@@ -54,7 +54,16 @@ To be released.
  -  Replaced Fedify's internal federation routing with
     *@fedify/uri-template* for stricter RFC 6570 URI Template expansion and
     matching.  The deprecated `Router` export from *@fedify/fedify* remains
-    available for compatibility.  [[#418]]
+    available for compatibility.  [[#418], [#758] by ChanHaeng Lee]
+
+ -  *Breaking change*: Tightened the deprecated `Router` shim from
+    *@fedify/fedify/federation*: `route()` now throws `RouterError` for
+    inputs that are not router paths (previously such inputs returned `null`
+    on non-match), and `trailingSlashInsensitive` is no longer mutable
+    post-construction (assignments after construction create an inert own
+    property; the flag must be passed to the constructor).  Callers should
+    validate inputs and pass options at construction, or migrate to `Router`
+    from *@fedify/uri-template*.  [[#418], [#758] by ChanHaeng Lee]
 
 [#316]: https://github.com/fedify-dev/fedify/issues/316
 [#418]: https://github.com/fedify-dev/fedify/issues/418
@@ -66,6 +75,7 @@ To be released.
 [#753]: https://github.com/fedify-dev/fedify/pull/753
 [#755]: https://github.com/fedify-dev/fedify/pull/755
 [#757]: https://github.com/fedify-dev/fedify/pull/757
+[#758]: https://github.com/fedify-dev/fedify/pull/758
 
 ### @fedify/fixture
 
@@ -85,8 +95,6 @@ To be released.
     matching.  This package replaces Fedify's previous direct use of
     *url-template* and *uri-template-router*.  [[#418], [#758] by ChanHaeng Lee]
 
-[#758]: https://github.com/fedify-dev/fedify/pull/758
-
 ### @fedify/amqp
 
  -  Added `AmqpMessageQueue.getDepth()` for reporting queued, ready, and