Blocked BitPacked: decode blocked bitwidths - #10202

Tracking Issue: #10167

## Summary

Adds a wire format for `BitPacked` arrays whose blocks each have their
own bit width, and makes it the in-memory ID, following
`fastlanes.for.v2` (#10135) and `DecimalByteParts`.

| ID | Role |
|---|---|
| `fastlanes.bitpacked` | Frozen wire format for global-width arrays.
Unchanged. |
| `fastlanes.bitpacked.v2` | In-memory ID, and the wire format for
per-block bit widths. |

`BitPackedPlugin` declares both serialized IDs and picks one per array
from `bit_widths()`:

- **Global width** writes `fastlanes.bitpacked` exactly as before.
Existing files read as before, and default writes stay byte-identical.
- **Blocked** writes `fastlanes.bitpacked.v2`.

The plugin moves into `bitpacking/plugin/`, laid out like FoR's:
`mod.rs` holds the IDs and the dispatch, `v1.rs` the v1 metadata and
serde, `v2.rs` the new format, and `tests.rs` the existing tests plus
new ones. The v1 code is moved unchanged, except that it writes
`bitpacked_v1_id()` rather than the plugin's ID.

## `fastlanes.bitpacked.v2` layout

- **Buffers**: the packed buffer, as in v1.
- **Children**: v1's children (patch indices, values and chunk offsets,
then validity), followed by the block offsets. The block offsets are
serialized like any other child, so the compressor can compress them.
- **Metadata**: a prost message holding `offset`, the block offsets'
unsigned `PType`, and the patches metadata. There is no bit width, and
the block offsets' length, `ceil((offset + len) / 1024) + 1`, is
derived.

Deserialization checks for one buffer and the expected number of
children, and validates the offset before deriving the block offsets'
length. Construction then checks the block offsets' dtype and length.
The decoder (#10202) bounds-checks the boundary values themselves.

## `BitPackedPatchedPlugin`

The experimental-patches plugin declares both IDs as well. It moves a v2
array's interior patches into a `Patched` wrapper just as it does for
v1, rebuilding the inner array with its block offsets instead of
panicking on a blocked array.

## Editions and compressor

`fastlanes.bitpacked.v2` is in no edition. Writers that enforce editions
reject it, and readers that predate it report an unknown encoding.
Nothing produces blocked arrays yet, so default writes don't change.
`BitPackingScheme::produced_encodings` now declares `bitpacked_v1_id()`,
since `BitPacked.id()` is the in-memory ID.

---------

Signed-off-by: Matt Katz <mhkatz97@gmail.com>