Add Problems panel for parse diagnostics

[fuzzzerd]

Problem

The status bar at the bottom of the main window summarises parse fidelity as a single line, e.g. "Parsed with 3 issue(s): UnknownStep (2), RoundTripValueMismatch (1)". The underlying ClipParseReport carries rich detail per diagnostic — kind, severity, XPath location, and message — but none of it surfaces. Users can see that something went wrong but not what or where, which makes triage hard.

Proposed fix

Expose the diagnostic list as a Problems panel that toggles from the existing status-bar text:

• The parse-fidelity summary in the status bar becomes a clickable toggle.
• A bottom-docked panel above the status bar (resizable via GridSplitter) lists every structural and semantic diagnostic with severity badge, kind, message, location, and line number.
• Parsing is augmented to preserve IXmlLineInfo; ClipParseDiagnostic gains nullable Line / Column fields.
• Selecting a row reveals the source XML around the line in a read-only snippet pane inside the panel (works for every clip type). When the active editor is a FallbackXmlEditor, it additionally scrolls and positions the caret to the line.

Out of scope

• Persisting panel open/closed state across sessions.
• Caret navigation inside the Script or Table editors (those don't display raw XML; the panel's snippet pane covers the line-level view for those clip types).
• Quick-fix actions on diagnostic rows.