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 <noreply@anthropic.com>

src/entities/mod.rs

@@ -1,7 +1,7 @@
 pub mod arc;
 pub mod attribute;
 pub mod circle;
-mod common;
+pub mod common;
 pub mod dimension;
 pub mod ellipse;
 pub mod hatch;