Move array serde from VTable to standalone ArrayPlugin impls - #10111

## Summary

Removes the blanket `impl<V: VTable> ArrayPlugin for V` and the
`serialize` / `deserialize` methods on `VTable`. Every serializable
encoding now implements `ArrayPlugin` explicitly on its vtable type, in
a `plugin.rs` child module next to its `impl VTable`. Redo of #10111 on
current develop.

## Changes

- `VTable` covers only in-memory behaviour; serde is `impl ArrayPlugin
for X`.
- Serde bodies are ported verbatim, so wire formats are unchanged.
- `read_ids` / `write_ids` default to `vec![self.id()]`, what the
blanket impl declared. Plugins with one wire ID no longer spell them
out, including `ALPPatchedPlugin`, `ScalarFnArrayPlugin` and the Python
stub.
- Inside an impl the wire ID is `ArrayPlugin::id(self)`; only the `id`
method itself delegates to `VTable::id`.
- `session.arrays().register(X)` is unchanged for every encoding that
was serializable.
- In-memory-only vtables lose the two stub methods and get no plugin:
Filter, Slice, Interleave, ScalarFn, ScalarFnValidity, ForeignArray,
PiecewiseSequence, Widen, Shared, test-harness vtables.
- `DecimalPlugin` (#10995) becomes `impl ArrayPlugin for Decimal`: one
wire ID, same legacy-node reads, no separate type. Its module,
`StoredDecimal` and tests stay where they are.
- Multi-ID plugins (`BitPackedPlugin`, `FoRPlugin`,
`DecimalBytePartsPlugin`) and `WidenPlugin` are untouched;
`ALPPatchedPlugin` now delegates to `ArrayPlugin::deserialize(&ALP,
..)`.
- `PythonVTable` keeps a stub plugin so the Python `register` API still
compiles.
- Vtable files that gained a `plugin.rs` child are directory modules
(`array.rs` becomes `array/mod.rs`, 14 pure renames). ALP's own impl and
`ALPPatchedPlugin` share `alp/array/plugin.rs`.
- Tests that called the vtable serde directly (ALP, ALPRD, Dict, List,
RunEnd, VarBin, RLE, serde.rs, unary, zstd buffers) go through
`ArrayPlugin` instead.

| | count |
|---|---|
| new `impl ArrayPlugin` (vortex-array) | 20 |
| new `impl ArrayPlugin` (encodings + python) | 17 |
| vtables that only lose the stub methods | 14 |

## API changes

- `VTable` loses `serialize` and `deserialize`. An external vtable that
was registered through the blanket impl must now `impl ArrayPlugin` to
be registrable.
- `ArrayPlugin::read_ids` / `write_ids` gain defaults of
`vec![self.id()]`.
- `vortex_array::arrays::DecimalPlugin` is removed; register `Decimal`
instead.

## Behaviour changes

- `PiecewiseSequence` is no longer registered by
`ArraySession::default()`; it never serialized, and its error is now "is
not registered for serialization".
- The writer seeds the footer's encoding list from the session registry,
so dropping `vortex.piecewise-sequence` makes every default-session file
48 bytes smaller. `test_stock_ticker_file_size` expectations move
accordingly (70060 → 70012, 69996 → 69948, 55144 → 55096).
- `DecimalByteParts`, `BitPacked` and `FoR` can no longer be registered
bare, so `bare_vtable_refuses_serde` and `vtable_serde_requires_plugin`
are removed.

## Testing

- `cargo clippy --all-targets --all-features -- -D warnings` and `cargo
+nightly-2026-09-10 fmt --check` on every touched crate (vortex-array,
vortex-file, vortex-layout, vortex-python and the 13 encodings crates).
- `cargo nextest run` on all touched crates plus vortex-ipc and
vortex-scan: 8981 passed, 0 failed; `cargo test --doc` on vortex-array,
fastlanes, decimal-byte-parts and alp.
- `cargo nextest run -p vortex-file -E
'test(test_stock_ticker_file_size)'` with and without `--features zstd`,
to derive the new sizes.

## Follow-up

- Pre-existing, unchanged here: the Patched, RLE, OnPair, ALPRD, FSST
and Zstd deserializers index children or buffers before checking their
counts, so a malformed node panics in `SerializedArray::child` instead
of returning an error. Worth hoisting the count checks in a separate PR.
- Fold `FoRPlugin` / `BitPackedPlugin` / `DecimalBytePartsPlugin` into
`impl ArrayPlugin for FoR` etc. if wanted.
- Overlaps with #11298, which makes the same change with inline impls
and trait defaults for `read_ids` / `write_ids`.

---------

Signed-off-by: Matt Katz <[email protected]>