feat: add HeaderVariables audit documentation

HEADER_AUDIT.md

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+# HeaderVariables Audit
+
+`acadrust::document::HeaderVariables` — 232 total.
+
+## Pass (no matching feature)
+
+- **ISOLINES** — contour line count on curved surfaces (no curved
+  wireframe generator yet)
+- **surface_u_density / surface_v_density / surface_type** — defaults
+  for SURFACE / REVSURF / EDGESURF commands (those commands don't
+  exist yet)
+- **lens_length / camera_height / camera_display** — perspective
+  camera (no perspective viewport yet)
+- **north_direction / latitude / longitude / timezone** — sun /
+  geographic data (no sun-shadow renderer)
+- **LIMMIN/LIMMAX** — drawing limits (no bounded-grid mode)
+- **INSBASE** — drawing's "save as block" anchor (no WBLOCK / "save
+  selection as block" flow)
+
+### Remaining (intentionally not surfaced)
+- ~70 `dim_*` defaults — render-time `DimStyle` reference is already
+  authoritative; these only seed a new DimStyle when a fresh DIMSTYLE
+  NEW flow is added.
+- `*_control_handle`, `*_dict_handle` — internal table-navigation
+  handles; direct `document.layers` / `document.dim_styles` / etc.
+  accessors bypass them. acadrust read/write preserves them.
+- AutoCAD UI prefs (`blip_mode`, `drag_mode`, `pick_style`, etc.) —
+  H7CAD has its own UI prefs and ignores AutoCAD's. Round-tripped.