feat: honour LUNITS / LUPREC unit formatting + INSUNITS scaling on insert

Property-panel coordinate display and block-insert scaling were both
hard-coded, ignoring the document's stored unit configuration.

- LUNITS / LUPREC: edit_prop() now formats values through a thread-local
  UnitContext seeded by refresh_properties(). Decimal / Scientific /
  Engineering / Architectural / Fractional all render correctly without
  threading the document handle through every entity properties builder.
- AUNITS / AUPREC: format_angle() helper available (decimal degrees, DMS,
  grad, rad). Callers that already format angular values via radians can
  switch over without touching the helper signatures.
- INSUNITS + MEASUREMENT: xref's source INSUNITS is carried onto the
  host BlockRecord.units during merge. commit_entity() then scales new
  INSERTs so 1 source-unit maps to the host's INSUNITS length. When
  either side is unitless (0) MEASUREMENT acts as the fallback
  (0=Imperial / inches, 1=Metric / mm), matching AutoCAD's rule.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/io/xref.rs

@@ -143,6 +143,13 @@ fn merge_xref_into_block(
 ) {
     let prefix = xref_block_name;
 
+    // Carry the xref's INSUNITS onto the host BlockRecord. Used at INSERT
+    // time so the inserted xref scales to the host's units (INSUNITS).
+    let src_insunits = xref_doc.header.insertion_units;
+    if let Some(br) = doc.block_records.iter_mut().find(|b| b.handle == br_handle) {
+        br.units = src_insunits;
+    }
+
     // ── Layers ──────────────────────────────────────────────────────────
     // Prefix every xref layer (including "0"). Entity layer references are
     // remapped below so the resolver finds the merged copy. Host layers