feat(dimstyle): block/linetype Handle cyclers for arrowheads & ext lines

The arrowhead-block (DIMBLK/DIMBLK1/DIMBLK2/DIMLDRBLK) and ext-line
linetype (DIMLTYPE/DIMLTEX1/DIMLTEX2) Handle fields are now editable in
the Dimension Style manager. Each shows the resolved block-record /
linetype name and a Cycle button that advances through the available
records (NULL = Default arrowhead / ByBlock linetype). Edits bump
geometry so dimensions re-render with the new arrowheads/linetypes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -1071,6 +1071,9 @@ pub enum Message {
     // Field edit messages:
     DsEdit(DsField, String),
     DsToggle(DsField),
+    /// Cycle a block/linetype Handle field on the selected dim style to the
+    /// next available block-record / linetype.
+    DsCycleHandle(&'static str),
     // ── Raster Image ──────────────────────────────────────────────────────
     /// Open file-picker dialog for IMAGE command (async).
     ImagePick,