executor: add AQSort (adaptive quicksort) for SortExec

[BornChanger]

What problem does this PR solve?

Issue Number: close #xxx

What changed and how does it work?

This introduces AQSort: an adaptive sort optimized for byte-orderable keys with
long shared prefixes, inspired by US7680791B2 (“Method for sorting data using
common prefix bytes”).

Why:

• SortExec currently relies on comparator-based sorting, which repeatedly calls
  per-type comparison functions and often re-scans shared key prefixes.
• ORDER BY keys in TiDB are frequently byte-encodable and prefix-heavy (e.g.
  composite keys / encoded datums), making them a good fit for a
  common-prefix-aware algorithm.

What’s included:

• New pkg/util/aqsort:

  • CPS-QS (common-prefix-skipping quicksort) + a radix partitioning step on the
    first byte after the common prefix.
  • Scratch-buffer reuse to reduce allocations.
  • Optional checkpoint callback to periodically check kill/cancel signals.
  • Introspective fallback to Go std sort when depth budget is exceeded.
  • NOTE: not stable; ordering is byte-lexicographic.

• SortExec integration (in-memory path):

  • Encode ORDER BY keys via codec.EncodeKey per row and sort Pair{Key, Row}
    using aqsort.PairSorter.
  • DESC handling is preserved by inverting encoded key bytes for DESC columns.
  • Parallel sort workers also support AQSort and reuse an arena for encoded keys
    to avoid per-key allocations where possible.

• User-facing enablement:

  • Add SESSION/GLOBAL sysvar tidb_enable_aqsort (default OFF) to opt into the
    AQSort execution path.
  • Keep the existing sortexec.SetAQSortEnabled toggle for benchmarking /
    experimentation; it still forces AQSort independent of the sysvar.

• Safety / observability:

  • Safe fallback: if AQSort key encoding fails, disable AQSort for the current
    SortExec and fall back to the original comparator-based sort so the query
    still succeeds.
  • Emit a one-time WARN log on first fallback with conn_id/executor_id and
    context (rows/worker_id) to aid debugging.
  • Add failpoint AQSortForceEncodeKeyError to deterministically test fallback.

Tests:

• Differential correctness tests ensuring AQSort and std sort produce identical
  row order for:
  • multi-type sort keys (string+collation, bytes, signed/unsigned ints, decimal,
    timestamp, duration, JSON), with NULLs and mixed ASC/DESC.
  • end-to-end SortExec (ORDER BY and window row_number sorting), including
    fallback behavior under forced encode errors.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Experimental: add an opt-in AQSort path for in-memory ORDER BY sorting; enable with `SET tidb_enable_aqsort=ON`.

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@BornChanger: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	3688970	link	true	/test check-dev
pull-build-next-gen	3688970	link	true	/test pull-build-next-gen
idc-jenkins-ci-tidb/build	3688970	link	true	/test build
pull-unit-test-next-gen	3688970	link	true	/test pull-unit-test-next-gen
idc-jenkins-ci-tidb/unit-test	3688970	link	true	/test unit-test

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