fix(composer): no LaTeX conversion inside a typed markdown fence or backtick span (#184 O3)

In markdown mode each paragraph line is serialised before parseBlockMD
joins them, so $x$ inside a ``` fence became data-mx-maths markup
inside the resulting <pre><code> (rendered as math in a code block).
Track fence state across lines and skip math for fenced lines and for
backtick code spans. Unit tests added; verified in the browser.

Also: scripts/dev-homeserver.sh enables MSC4140 delayed events so
scheduled messages work locally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

scripts/dev-homeserver.sh

@@ -43,6 +43,10 @@ rc_joins: { local: { per_second: 1000, burst_count: 10000 }, remote: { per_secon
 rc_presence: { per_user: { per_second: 1000, burst_count: 10000 } }
 max_upload_size: 50M
 suppress_key_server_warning: true
+# scheduled messages (MSC4140 delayed events)
+max_event_delay_duration: 24h
+experimental_features:
+  msc4140_enabled: true
 """
 open(p, "w").write(s)
 PY