Add read-update-write benchmark

Update the README with a new benchmark group and performance results.
Include new data files and update the Rust benchmark implementation.
Regenerate UPB bindings and fix a data path in the C benchmark runner.

README.md

@@ -147,12 +147,13 @@ end offsets of the field range are recorded to bound iteration efficiently.
 Two benchmark suites share the same binary data files and the same four
 measurement groups:
 
-| Group           | What is timed                                           |
-| --------------- | ------------------------------------------------------- |
-| `shallow_parse` | Become ready to read any field (one scan / full decode) |
-| `deep_parse`    | Walk the full tree: Campaign → Operations → Hackers     |
-| `field_access`  | Read individual fields on an already-parsed message     |
-| `iterate`       | Count top-level and nested repeated fields              |
+| Group               | What is timed                                           |
+| ------------------- | ------------------------------------------------------- |
+| `shallow_parse`     | Become ready to read any field (one scan / full decode) |
+| `deep_parse`        | Walk the full tree: Campaign → Operations → Hackers     |
+| `field_access`      | Read individual fields on an already-parsed message     |
+| `iterate`           | Count top-level and nested repeated fields              |
+| `read_update_write` | Parse, update a field, and serialize back to a buffer   |
 
 ### 1 — Generate the shared data files (do this once)
 
@@ -267,6 +268,18 @@ criterion medians; C/upb times are the custom runner's mean over ≥ 0.5 s.
 > `shallow_parse`. `count_all_crew` also parses each `Operation` sub-message;
 > roto's per-level scans remain cheaper than upb's full decode.
 
+#### `read_update_write` — parse, update a field, and serialize back to a buffer
+
+| Size   |     Bytes | roto (ns) |    upb (ns) | roto speedup |
+| ------ | --------: | --------: | ----------: | -----------: |
+| tiny   |       588 |     153.8 |     1,120.3 |     **7.3×** |
+| small  |    20,265 |   1,301.8 |    42,089.6 |    **32.3×** |
+| medium | 2,071,053 | 302,090.0 | 9,233,397.9 |    **30.5×** |
+
+> roto's `with()` method allows copying fields directly from the original binary
+> without decoding them, making the update process extremely efficient. upb must
+> fully parse the message into structs and then re-serialize the entire tree.
+
 ### Interpreting the comparison
 
 The two libraries have fundamentally different models: