docs: document dump importers, add design doc and benchmark script

- ADMIN_API.md: dump_url semantics and the new dump_importer field.
- USER_GUIDE.md: creating a database from a SQLite dump, importer selection
  and the --dump-import-* flags.
- STREAMING_DUMP_IMPORT_DESIGN.md: the design the implementation follows,
  with 'as built' notes where it was refined (422 for unknown importer,
  size limit also applies to terminated statements, exact UTF-8 error
  position, DDL text differences between importers) and first results.
- scripts/bench-dump-import.sh: runs both importers against one server,
  samples RSS, compares exported data and integrity_check.

Author: tszymczyszyn-shopify

docs/ADMIN_API.md

@@ -21,9 +21,30 @@ body:
 ```json
 {
     "dump_url"?: string,
+    "dump_importer"?: "buffered" | "streaming",
 }
 ```
 
+`dump_url` initializes the new namespace from a SQLite SQL dump (`sqlite3 db .dump` or this
+server's `GET /dump` output). Supported schemes are `file:` (an absolute path on the server
+host) and `http(s):`. The dump must run inside a transaction and end with `COMMIT`; `ATTACH` is
+rejected.
+
+`dump_importer` selects how the dump is loaded (it requires `dump_url`):
+
+- `buffered` (historical): the whole dump is read into memory, parsed, then executed. Memory
+  usage is proportional to the dump size.
+- `streaming`: statements are framed with `sqlite3_complete()` and executed while the dump is
+  still being read. Memory usage is bounded by the server's queue settings plus the largest
+  single statement (see `--dump-import-*` flags); a statement larger than
+  `--dump-import-max-statement-size` is rejected with `413`.
+
+When omitted, the server's `--dump-importer` setting (`SQLD_DUMP_IMPORTER`, default `buffered`)
+applies. Both importers produce the same data; the streaming importer additionally stores the
+schema SQL exactly as written in the dump, whereas the buffered importer stores the parser's
+normalized rendering. A dump whose *data* contains the word "attach" is rejected by the buffered
+importer (substring check) but accepted by the streaming one (statement-level check).
+
 ```HTTP
 DELETE /v1/namespaces/:namespace
 ```