* fa: fix FlashQKV early-termination causing S=0 assertion with --parallel N>1
The backward-scan optimization in compute_helper/compute_helper_q checks
only one mask position per k_step block on the last query row (q_step-1)
to find where valid KV entries end. When q_step > 1 and different query
rows have non-overlapping valid KV regions (multi-slot / --parallel N>1),
the scan on the last row's mask can miss blocks that contain valid entries
for earlier rows. This causes those rows to accumulate S=0, triggering
the GGML_ASSERT(S > 0) in normalize_and_store_1row.
Fix: remove the early-termination scan at all 4 sites and iterate all
nk1/k_step blocks unconditionally. The mask already handles correctness:
fully-masked blocks produce smax=-inf and skip V accumulation, so the
performance cost is minimal for TG (small nq1) and acceptable for PP.
Fixes#809
* fa: refactor multi-slot mask fix into mask_effective_nk1() helper
Replace 4× inlined early-termination scans with a shared helper that
computes the effective K boundary by scanning ALL query mask rows
(union-of-masks). This is the minimal fix for multi-slot parallel
inference where different slots have different sequence lengths.
The helper returns the k_step-aligned boundary covering the longest
active sequence across all rows, preserving single-slot performance
(single row = same boundary as before).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---------
Co-authored-by: Turbomen008 <Turbomen008@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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