docs(rust): record MT reset and completion seams

Update the migration boundary document to record the two latest multithreaded
policy moves. Rust now owns serial-reset LDM normalization, table sizing, and
publication plus final completion trace/reset ordering, while C still owns the
private job/context state and callbacks.

Test Plan:
- `git diff --cached --check` -- passed.
- Capped native build and full original test target were run before this
  documentation-only commit and passed.

rust/README.md

@@ -108,9 +108,10 @@ zstd ABI:
     accounting.  Rust also owns frame-block preparation ordering and the MT
     serial turn/skip policy, including LDM-before-checksum sequencing; C
     callbacks retain the private match-state window, workspace operations,
-    synchronization, LDM state, and checksum state. Rust also owns the normal
-    job-table teardown order, while C retains the private job storage and
-    synchronization callbacks.
+    synchronization, LDM state, and checksum state. Rust also owns serial-reset
+    LDM normalization/table sizing/publication, final MT completion ordering,
+    and normal job-table teardown; C retains the private job storage, context
+    mutation, and synchronization callbacks.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
 - Legacy decoding
@@ -163,8 +164,9 @@ CLI zstd compression stream loop, scalar adaptive decisions, and the
 shared- and separate-destination multi-file compression schedulers,
 shared- and separate-destination decompression scheduling,
 single-threaded stream initialization and the buffered/stable stream state
-machine, MT stream initialization, MT outer scheduling and flush policy, MT
-compression-job stage sequencing and error flow, MT frame-progression job
+machine, MT stream initialization, MT serial-reset LDM policy and publication,
+MT outer scheduling and flush policy, MT completion trace/reset ordering,
+MT compression-job stage sequencing and error flow, MT frame-progression job
 aggregation, frame-block preparation ordering, MT serial turn/skip and
 LDM/checksum sequencing, overflow-correction policy/order, public sequence-API
 orchestration, sequence-store and block policy, external-producer invocation