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

[gargiulofrancesco]

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.

[github-actions]

Polar Signals Profiling Results

Latest Run

Status	Commit	Job	Attempt	Link
🟢 Done	ee01ecc		1	Explore Profiling Data

Previous Runs (2)

Status	Commit	Job	Attempt	Link
🟢 Done	b061163		1	Explore Profiling Data
🟢 Done	fc8ced8		1	Explore Profiling Data

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[codspeed-hq]

Merging this PR will not alter performance

✅ 1796 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing ji/onpair-non-expiremental (ee01ecc) with develop (5746d55)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

Benchmarks: Vortex queries 📖

Verdict: No clear signal (low confidence)
Attributed Vortex impact: +2.0%
Engines: DataFusion No clear signal (+7.1%, low confidence) · DuckDB No clear signal (-2.9%, low confidence)
Vortex (geomean): 1.009x ➖
Parquet (geomean): 0.999x ➖
Shifts: Parquet (control) -0.1% · Median polish -0.1%

How to read Verdict and Engines

• Verdict: Overall PR-level signal after subtracting baseline drift estimated from Parquet control rows. It can be Likely improvement, Likely regression, or No clear signal.
• Engines: Per-engine attribution. DataFusion is compared against DataFusion/Parquet controls; DuckDB is compared against DuckDB/Parquet controls. This answers whether each engine improved or regressed independently.
• Confidence: Based on directional consistency, share of rows above the noise floor, and control-run noise.

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datafusion / vortex-file-compressed (1.067x ➖, 0↑ 0↓)

name	PR ee01ecc (ns)	base 5746d55 (ns)	ratio (PR/base)
vortex_q00/datafusion:vortex-file-compressed	10019152	9410240	1.06
vortex_q01/datafusion:vortex-file-compressed	6431205	6017158	1.07

datafusion / parquet (0.996x ➖, 0↑ 0↓)

name	PR ee01ecc (ns)	base 5746d55 (ns)	ratio (PR/base)
vortex_q00/datafusion:parquet	20682192	21234103	0.97
vortex_q01/datafusion:parquet	4748623	4660522	1.02

duckdb / vortex-file-compressed (0.973x ➖, 0↑ 0↓)

name	PR ee01ecc (ns)	base 5746d55 (ns)	ratio (PR/base)
vortex_q00/duckdb:vortex-file-compressed	10435104	10242605	1.02
vortex_q01/duckdb:vortex-file-compressed	5987331	6436835	0.93

duckdb / parquet (1.002x ➖, 0↑ 0↓)

name	PR ee01ecc (ns)	base 5746d55 (ns)	ratio (PR/base)
vortex_q00/duckdb:parquet	23472627	23381736	1.00
vortex_q01/duckdb:parquet	9443826	9440194	1.00

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No file size changes detected.