feat(onpair): promote OnPair to a stable read-only encoding (#8872)

## Rationale for this change

OnPair has been living under `encodings/experimental/` while it
stabilized. We now
want OnPair-encoded files to be readable by default, without yet
committing to writing
them. This PR promotes OnPair to a stable encoding but enables it on the
**read path
only** — the compressor/writer stays gated behind the
`unstable_encodings` feature so
we can turn on writing in a follow-up.

## What changes are included in this PR?

- Move the crate from `encodings/experimental/onpair` →
`encodings/onpair` (100% file
renames) and drop it from the "Experimental encodings" group in the
workspace
  `Cargo.toml`.
- Register the OnPair reader unconditionally in
`register_default_encodings`, so any
  session can decode OnPair arrays and push down filters against them.
- Keep the writer/compressor disabled by default:
  - `ALLOWED_ENCODINGS` (file write strategy) only inserts OnPair under
    `#[cfg(feature = "unstable_encodings")]`.
- btrblocks `ALL_SCHEMES` and the `onpair` scheme module are gated
behind
`unstable_encodings`, so the default compressor never selects OnPair.
- Tests:
- New `default_writer_does_not_allow_onpair` asserts OnPair is absent
from the default
write allowlist; the reader-registration test asserts OnPair is
registered with a
    Filter execute-parent kernel.
- btrblocks scheme-selection tests that exercise writing are gated
behind
    `unstable_encodings` and renamed to make the scope explicit
    (`test_unstable_all_schemes_includes_onpair`,
    `test_unstable_default_btrblocks_compressor_selects_onpair`).

Verified locally:
- Default (reader-only) config: `vortex-onpair`, `vortex-btrblocks`,
`vortex-file` —
  169 tests passed.
- `unstable_encodings` (writer-enabled) config: 176 tests passed (the
extra 7 are the
  feature-gated write-path tests).
- `cargo clippy --all-targets --all-features` (affected crates) clean;
  `cargo +nightly fmt --all` clean; `git diff --check` clean.

## What APIs are changed? Are there any user-facing changes?

- The `vortex-onpair` crate moves to `encodings/onpair` (path only;
crate name and
  version are unchanged).
- User-facing behavior change: OnPair-encoded files are now **readable
by default**.
Writing OnPair still requires the `unstable_encodings` feature — no
default write
  behavior changes.

---------

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>
Co-authored-by: Joe Isaacs <joe.isaacs@live.co.uk>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

Cargo.toml

@@ -59,8 +59,7 @@ members = [
     "encodings/zstd",
     "encodings/bytebool",
     "encodings/parquet-variant",
-    # Experimental encodings
-    "encodings/experimental/onpair",
+    "encodings/onpair",
     # Benchmarks
     "benchmarks/lance-bench",
     "benchmarks/compress-bench",
@@ -307,7 +306,7 @@ vortex-json = { version = "0.1.0", path = "./vortex-json", default-features = fa
 vortex-layout = { version = "0.1.0", path = "./vortex-layout", default-features = false }
 vortex-mask = { version = "0.1.0", path = "./vortex-mask", default-features = false }
 vortex-metrics = { version = "0.1.0", path = "./vortex-metrics", default-features = false }
-vortex-onpair = { version = "0.1.0", path = "./encodings/experimental/onpair", default-features = false }
+vortex-onpair = { version = "0.1.0", path = "./encodings/onpair", default-features = false }
 vortex-parquet-variant = { version = "0.1.0", path = "./encodings/parquet-variant" }
 vortex-pco = { version = "0.1.0", path = "./encodings/pco", default-features = false }
 vortex-proto = { version = "0.1.0", path = "./vortex-proto", default-features = false }