[opt](hive) vectorize single-char hive text split and cache KMP prefix table

[Baymine]

Proposed changes

Issue Number: close #66163

Problem Summary

HiveTextFieldSplitter splits every Hive text line on the hot scan path. This PR removes two avoidable costs, both behavior-preserving (split results are identical):

1. Single-char separator, no escape char (common case). The escape_char == 0 branch of _split_field_single_char scanned the line byte-by-byte. It now uses memchr to locate each separator, letting glibc use its vectorized search. The escape path is unchanged and still uses is_hive_text_separator_escaped.

2. Multi-char separator (MultiDelimitSerDe). _split_field_multi_char rebuilt the KMP prefix table from the separator on every line. It is now computed once in the constructor (_kmp_next) and reused per call. This is a strict win (no downside).

Benchmark (memchr vs for+if, single-char path)

There is an existing note in new_plain_text_line_reader.cpp that for + if beats memchr "under normal short fields case". A micro-benchmark of the split loop (plain -O2, 4M iters/case) shows the tradeoff precisely:

field width	for+if	memchr	speedup
3 bytes (tiny)	182 ns	192 ns	0.95x
6 bytes	210 ns	189 ns	1.11x
20 bytes	283 ns	175 ns	1.62x
100 bytes	958 ns	175 ns	5.48x
400 bytes	949 ns	58 ns	16.3x

memchr wins for field widths >= ~6 bytes (the normal case for real Hive columns: strings, numbers, timestamps, paths) and is dramatically faster for wide values. The only regression is ~5% for pathologically tiny 1-3 byte fields, which matches the existing note. If reviewers prefer, the fast path can be gated behind a small line-length threshold to keep for + if for tiny lines; happy to add that.

Release note

None

Check List (For committer)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Reason:

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For author)

• Unit Test: be/test/format/text/hive_text_field_splitter_test.cpp gains 5 cases covering the memchr fast-path boundaries (Hive \x01 default separator, long fields spanning SIMD chunks, consecutive/leading/trailing separators, multi-byte UTF-8), the escape vs no-escape divergence, the cached KMP table reused across rows, and the multi-char escape boundary conditions. HiveTextFieldSplitterTest 10/10 pass locally.

• Manual test: micro-benchmark of the split loop (results above).

• I have unit-tested and manually tested this change.

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Baymine]

run buildall

[Baymine]

Closing this: a more rigorous benchmark (interleaved runs, pinned core) shows the memchr fast path regresses ~10-14% for sub-6-byte fields (break-even at ~6 bytes, only a clear win at >=20 bytes), which confirms the existing 'for+if beats memchr for short fields' note. Will revisit with a width-gated hybrid if pursued.