Map SectionSymbol, the last unmapped kind

points is canonical; the host derives the point counts and refreshes the
end/tick/label projections with sync_display_fields when it changes, and
validates that style and view-representation references are existing class
objects. The lifecycle test passes every gate in DWG; DXF reopens the symbol
as an unknown entity because cadcodec dispatches SECTIONLINE only inside
blocks (pinned by a canary and already in the cadcodec write-up).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/cadcodec-reader-gaps.md

@@ -151,7 +151,8 @@ reopens as a different, opaque entity kind instead of `ViewBorder`. DWG restores
 the typed record. The same applies to `SectionSymbol`.
 
 **Evidence:** `audit_python_view_border_lifecycle_over_real_ipc` (canary
-`expect_edited_dxf: "wrong kind"`).
+`expect_edited_dxf: "wrong kind"`) and `audit_python_section_symbol_lifecycle_over_real_ipc`
+(canary `wrong kind Unknown`).
 
 **Suggested fix:** add the two arms to the ENTITIES dispatch, calling
 `read_view_border_dxf` and `read_section_symbol_dxf`.