docs(rust): document sequence and adaptive boundaries

Document that public sequence API orchestration and scalar CLI adaptive
decisions are Rust-owned while private codec/context state and mutation
callbacks remain in C.

Test Plan:
- git diff --cached --check
- documentation-only boundary update; code validation is covered by
  3b989ab3e and b04893799

Commit is intentionally unsigned because GPG pinentry hangs in this
non-interactive environment.

rust/README.md

@@ -59,6 +59,8 @@ zstd ABI:
     stream-initialization policy: dictionary selection, parameter resolution,
     initial buffer sizing, and the ordered setup decisions are projected into
     Rust while C retains the private contexts and mutation callbacks.
+    The public sequence APIs likewise use Rust-owned validation, frame-header,
+    checksum, and output-accounting orchestration around C-owned block state.
   - `zstd_compress_params` owns the compression-level tables (formerly
     `clevels.h`), parameter bounds, clamping, validation, table selection,
     source/dictionary adjustment, and match-state/CDict size estimation.
@@ -112,9 +114,10 @@ zstd ABI:
     so library builds do not acquire program-only dependencies.  The C
     `fileio` layer retains the format-specific codec callbacks, private
     asynchronous-pool adapters, metadata, zstd codec/error mapping, and
-    adaptive compression policy, while Rust owns the mixed-format
-    probe/dispatch loop, zstd stream-compression I/O loop, optional-format
-    decompression loops, and decompression result policy/final accounting.
+    adaptive-policy integration, while Rust owns the mixed-format probe/dispatch
+    loop, zstd stream-compression I/O loop, scalar adaptive decisions,
+    optional-format decompression loops, and decompression result policy/final
+    accounting.
     Rust already owns the file preference policy, filename decisions,
     source/destination opening, dictionary buffers, asynchronous I/O pools,
     and pass-through copy leaf.
@@ -128,10 +131,11 @@ zstd ABI:
     tests do not compile.
 
 Dictionary-ingestion dispatch, CCtx dictionary/prefix attachment dispatch, the
-CLI zstd compression stream loop, 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, sequence-store and block policy,
+CLI zstd compression stream loop and scalar adaptive decisions,
+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, public sequence-API
+orchestration, sequence-store and block policy,
 external-sequence/literals block loop, optional-format decompression loops,
 block decoder wrappers, and decompression result policy now run in Rust. CDict
 lifecycle, the private dictionary-content loader, reset policy, private