Optimize concat/concat_ws scalar path by pre-allocating memory

[lyne7-sc]

Which issue does this PR close?

• Closes #.

Rationale for this change

This PR improves performance by:

• Using exact Utf8View byte size (sum of data buffers) instead of row-based approximation.
• Building results via .concat()/.join(sep) on a pre-allocated Vec<&str> to avoid String reallocations.

Benchmark

Case	Before	After	Change
concat_ws_scalar/8	299.18 ns	233.18 ns	-21.93%
concat_ws_scalar/32	327.53 ns	251.44 ns	-23.23%
concat_ws_scalar/128	405.80 ns	271.27 ns	-33.15%
concat_ws_scalar/4096	976.02 ns	791.33 ns	-18.92%
concat_scalar/8	248.71 ns	221.24 ns	-11.05%
concat_scalar/32	284.26 ns	240.53 ns	-15.39%
concat_scalar/128	301.91 ns	257.61 ns	-14.67%
concat_scalar/4096	916.68 ns	805.33 ns	-12.15%

What changes are included in this PR?

Performance optimization for concat and concat_ws functions scalar path.

Are these changes tested?

• Existing unit and integration tests pass.
• New benchmarks added to verify performance improvement.

Are there any user-facing changes?

No. It's a pure performance optimization.

[Omega359]

🤖 ./gh_compare_branch_bench.sh Benchmark Script Running
Linux fedora 6.17.12-300.fc43.x86_64 #1 SMP PREEMPT_DYNAMIC Sat Dec 13 05:06:24 UTC 2025 x86_64 GNU/Linux
Comparing perf/concat (301ab2e) to 83ed192 diff
BENCH_NAME=concat
BENCH_COMMAND=cargo bench --bench concat
BENCH_FILTER=
BENCH_BRANCH_NAME=perf_concat
Results will be posted here when complete

[Omega359]

🤖 ./gh_compare_branch_bench.sh Benchmark Script Running
Linux fedora 6.17.12-300.fc43.x86_64 #1 SMP PREEMPT_DYNAMIC Sat Dec 13 05:06:24 UTC 2025 x86_64 GNU/Linux
Comparing perf/concat (301ab2e) to 83ed192 diff
BENCH_NAME=concat
BENCH_COMMAND=cargo bench --bench concat
BENCH_FILTER=
BENCH_BRANCH_NAME=perf_concat
Results will be posted here when complete

[Omega359]

🤖: Benchmark completed

Details

group                            main                                   perf_concat
-----                            ----                                   -----------
concat function/concat/1024      1.03      9.3±0.85µs        ? ?/sec    1.00      9.0±0.29µs        ? ?/sec
concat function/concat/4096      1.00     35.1±0.71µs        ? ?/sec    1.00     35.1±1.01µs        ? ?/sec
concat function/concat/8192      1.01     69.9±3.74µs        ? ?/sec    1.00     69.6±0.95µs        ? ?/sec
concat function/concat/scalar                                           1.00     32.5±0.13µs        ? ?/sec