feat(sdk): support batched prompts in geniex-bench via --- separator

[RemiliaForever (RemiliaForever)]

Summary

geniex-bench's --prompt-file can now hold multiple prompts in one file, separated by a line that is exactly --- (leading/trailing whitespace on that line is ignored).

• Each ----delimited segment runs as its own prompt, with the KV cache reset between segments (even under --no-reset-between-runs).
• stdout marks segment boundaries with a [sep ] prompt i/n line (the previous [gen ]-prefixed header is gone, so it no longer collides with generated text).
• A file with no --- line stays a single prompt, so timing and --accuracy runs share one code path — no branching on mode.
• Empty / whitespace-only segments (leading, trailing, or back-to-back ---) are skipped; a file that is only separators errors out.

Only sdk/benchmark/benchmark.c changes; benchmark.c is not a public SDK header, so no FFI update is required. Existing QDC entry scripts pass single prose prompt files with no --- line, so their behavior is unchanged.

Test plan

• gcc -Wall -Wextra compiles and links cleanly against pkg-geniex
• clang-format --Werror clean
• End-to-end with Qwen3-0.6B (llama_cpp, cpu): multi-segment split produces distinct, coherent per-segment output
• Boundary cases: 3 segments, leading ---, back-to-back ---, trailing ---, no --- (single prompt), multi-line segment, whitespace-padded ---, only-separators (errors)
• On-device QDC bench run (llama_cpp + qairt) still green