Document record-structure dictionary counts on describe_table

reference/operations-api/operations.md

@@ -69,6 +69,35 @@ Returns the definition of a specific table.
 { "operation": "describe_table", "table": "dog", "database": "dev" }
 ```
 
+Alongside the schema, the response carries the size of the table's **record-structure dictionaries**
+— the physical record layouts Harper has seen for this table:
+
+| Field                      | Description                                                                          |
+| -------------------------- | ------------------------------------------------------------------------------------ |
+| `typed_structures_enabled` | Whether random-access (typed) encoding is enabled for this table                     |
+| `typed_structure_count`    | Structures in the random-access dictionary                                           |
+| `typed_structure_limit`    | Bound past which novel record shapes are stored without random-access field encoding |
+| `classic_structure_count`  | Structures in the classic named-record dictionary                                    |
+
+A structure is minted per distinct _shape_, where shape means the ordered list of fields plus each
+field's value width class — so `{a, b}` and `{b, a}` are different shapes, and so are `{v: 1}` and
+`{v: 70000}`. Dictionary size therefore tracks the variety of shapes a table has ever written, not
+its column count, and it only grows: structures are never pruned, because stored records,
+transaction-log entries, and replication backlogs all reference them by id.
+
+`storage.randomAccessFields` defaults to off, so `typed_structures_enabled: false` with
+`typed_structure_count: 0` is the normal state for most tables — that is typed encoding being
+disabled, not spare headroom. Where it is enabled, reaching `typed_structure_limit` is not an error:
+records with novel shapes past that point still write and read correctly, but are stored without
+random-access field encoding, which makes reading individual fields of large records slower. Harper
+logs a warning when a thread first observes the bound, but that depends on which thread served the
+writes and whether it has loaded the dictionary — the counts here are the reliable signal.
+
+To keep the dictionary small, write records in a consistent field order, avoid making the _set_ of
+present fields vary per write (nest volatile or optional fields inside one sub-object rather than
+adding and removing top-level fields), and expect a small fixed number of extra shapes from numeric
+fields whose values cross a width boundary.
+
 ### `create_database`
 
 Creates a new database.