feat(mtext): evaluate dynamic text fields at render time

Re-evaluate AcDbField-driven MTEXT — date/login/document-property fields, table
formulas and object properties — against the current system instead of showing
the frozen last-save cache. The evaluation engine lives in `acadrust::fields`;
OCS supplies only the environment (system clock, OS login, env vars) through a
FieldContext implementation, and hooks the resolved text into MTEXT rendering,
falling back to the cached value for anything it can't evaluate.

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

src/entities/mod.rs

@@ -5,6 +5,7 @@ pub mod common;
 pub mod dim_override;
 pub mod dimension;
 pub mod ellipse;
+pub mod field;
 pub mod hatch;
 pub mod helix;
 pub mod insert;