Replace the logical codec's object registry with durable metadata

`examples/datafusion-ffi-example/src/logical_extension_codec.rs` parks live table providers in a process-global `HashMap` and encodes an integer token into it. Encoding inserts, decoding removes, so the same bytes cannot be decoded twice, one plan cannot fan out to several readers, and a plan that never reaches a decoder keeps its provider alive for the life of the process. `extension-guide/codecs.md` tells authors not to do this.

Unlike the physical codec in the same crate (see the quarantine sub-issue), this one is fixable: `try_encode_table_provider` at line 148 claims `node.downcast_ref::()`, which is narrow, and a `MemTable` is fully describable by its schema and batches.

**Pattern to copy:** `examples/distributed/storage-library/src/codec.rs` — same Arrow IPC technique, same error convention (`internal_datafusion_err!` on encode, since this process holds the object; `exec_datafusion_err!` on decode, since those are foreign bytes).

**Verified prerequisites:** `MemTable.batches` is `pub` (`datafusion-catalog/src/memory/table.rs:69`), typed `Vec` where `PartitionData = Arc>>`. `MemTable::try_new` rejects zero partitions (`table.rs:84`). `arrow` is already a dependency with IPC available, so no `Cargo.toml` change.

Proposed wire format, keeping the per-instance prefix the dispatch tests rely on:

```
| b"MEMTBL1" | u32 LE n_partitions | { u32 LE ipc_len | ipc stream }*
```

One stream per partition, because `MemTable` partition boundaries become output partitions. A stream carries its schema even when empty, so an empty partition round-trips.

Two traps worth writing down before someone hits them:

- Use `try_read()` on each partition lock, **not** `blocking_read()`. The FFI codec runs with a tokio runtime handle installed, and `blocking_read` panics in that context.
- On decode, build `MemTable::try_new` from the **IPC** schema, not the `schema: SchemaRef` argument. `try_new` validates `schema.contains(&batch.schema())`, so metadata drift would surface as a spurious mismatch. This is the opposite choice from `storage-library/src/codec.rs:376-380`, which must honour the plan's schema because it re-reads files from disk; here the batches *are* the payload. Worth a comment noting the contrast, since the two codecs otherwise look alike.

**Done when:** the registry, `token_id()`, and the `HashMap`/`Mutex`/`OnceLock`/`AtomicU64` imports are gone; the struct field `token` is renamed `provider_prefix` to match the Python kwarg that already uses that name; and `grep -in token` over the file returns nothing.

**Tests:** of 19 tests in `python/tests/_test_logical_extension_codec.py`, one changes. `test_installing_a_codec_cannot_hijack_an_earlier_codecs_objects` asserts `len(before) == len(after)` with a comment about tokens being minted per encode; that comment becomes false and the assertion becomes weaker than reality, so it should become `assert before == after`. Add one test for the property the guide claims and nothing currently covers: encode once, decode twice on one session, assert both produce the same rows. All 47 tests in the planner crate should be unaffected — every assertion there is on call counters, never on payload shape.

Start in examples/datafusion-ffi-example/src/logical_extension_codec.rs at try_encode_table_provider and compare the IPC approach in examples/distributed/storage-library/src/codec.rs. Update python/tests/_test_logical_extension_codec.py, including the earlier-codec assertion and a decode-twice test. Done means the registry and token code are gone, provider_prefix is used, the specified tests pass, and grep -in token returns nothing.

索引モデルが issue の本文から書いたものです。