feat(plugin): name-addressed XRecord CRUD on the HostApi (API v8)

Add create/read/update/delete/list for XRecord objects under
named-object-dictionary paths to the plugin HostApi, with five additive
PluginRequest/PluginResponse variants, dispatch through the shared
handler, in-process HostSession methods and both out-of-process proxies.

- dictionary paths walk from the NOD root; create seeds missing
  dictionaries (ensure_image_dictionary bootstrap) and refuses existing
  names; update refuses missing ones; refusals precede the undo step
- entry group codes validated (1..=369, never 5/105); payload content is
  the plugin's own
- each mutating call is one undo step: the pending history snapshot
  restores the objects map byte-exactly, auto-created dictionaries
  included (tested)
- typed entries survive DWG and DXF write/reload round-trips (tested);
  update drops verbatim raw bytes so saves emit the new content
- bump API_VERSION 7 -> 8 with the build.rs version_info in lockstep,
  mirroring the v7 addition; MIN_SUPPORTED stays 3

docs/plugin-host-model.md

@@ -26,6 +26,7 @@ without it keeps working, and the default `HostApi` methods refuse.
 | Selection | `selection`, `set_selection` | Ordered, tab-scoped, exact (missing or duplicate handles reject the request). |
 | Solids | `solid_operation` | Kernel-backed create, transform, booleans, extrude, region and surface from a profile, picture embedding. Every result is verified to lift back from its ACIS payload without loss; a refusal leaves the operands untouched. |
 | Tables | `table_operation` | Layers, text and dimension styles, blocks and their contents, linetypes, layouts (see below). |
+| Objects | `create_xrecord`, `read_xrecord`, `update_xrecord`, `delete_xrecord`, `list_xrecords` | Name-addressed XRecords under the drawing's named-object dictionary (API v8, see below): a non-graphical record carrier with strict create-vs-update semantics and one undo step per change. |
 | Commands | `run_command` | Drives the real OCS command one step at a time (see below). |
 | Settings | `system_variable`, `set_system_variable` | `CLAYER`, `SNAPANG`, and a read-only `CTAB` (the current layout). |
 
@@ -85,6 +86,22 @@ recorded, so a failure changes nothing and leaves the history alone.
 Style and layout operations act on the active document, because they use the
 application's shared style and view machinery.
 
+## XRecords (API v8)
+
+`create_xrecord`, `read_xrecord`, `update_xrecord`, `delete_xrecord` and
+`list_xrecords` address XRecords by name under a dictionary path walked from the
+named-object dictionary root (an empty path is the root itself); missing
+dictionaries are created on create only, so the first call seeds a plugin's
+namespace. The payload is the codec's typed `XRecordEntry` pairs — the host puts
+no opinion on the content. Names and path fragments are symbol-validated, and
+dictionary addressing follows the codec's case-insensitive lookup. Create never
+overwrites and update never creates; each refusal is decided before the undo step
+is recorded, so a failure changes nothing, and each change is one undo step whose
+undo restores the objects map byte-exactly, the auto-created dictionaries
+included. `list_xrecords` reports only the keys that address XRecords, in the
+dictionary's own order, so audits need no prior knowledge. Not yet mapped to
+Python.
+
 ## Commands
 
 `run_command` takes a `CommandRequest` (`Run`, `Start`, `Point`, `Text`, `Token`,