metal: add opt-in V skip for negligible attention weights

[TheTom]

Summary

Adds an opt-in optimization to the Metal flash attention vec kernel that
skips V dequantization and accumulation for positions where the post-softmax
attention weight is below 1e-6.

Reduces wasted dequant + FMA work when attention is sparse.

Gated by GGML_METAL_FA_SKIP_V=1 environment variable (default: off).
No behavior change unless explicitly enabled.

Changes

• ggml-metal.metal: threshold check in quantized V accumulation loop
• ggml-metal-device.m: env var gating + log message

Validation (Apple M5 Max, Qwen3.5-35B-A3B Q8_0)

Perplexity (wikitext-103, 32K context, 50 chunks):

SKIP_V=0: 7.0638 ± 0.021
SKIP_V=1: 7.0638 ± 0.021

Delta: 0.000

Decode (llama-bench, q8_0):

SKIP_V=0: 85.15 tok/s (tg128), 1142.93 tok/s (pp32768+tg128)
SKIP_V=1: 85.04 tok/s (tg128), 1144.78 tok/s (pp32768+tg128)

Within noise. No regression observed.

Scope

• Metal only (no CUDA changes)
• Applies to quantized V path in FA vec kernel
• Default off (opt-in via env var)

Notes

• Threshold (1e-6) produced identical PPL across tested values (1e-4 to 1e-8)
  in this setup
• Effect depends on V dequant cost; minimal difference observed for q8_0
• Also tested on q4_0 KV cache with identical ON/OFF PPL

[TheTom]

@ggerganov @slaren Gentle ping on this one. Small PR (2 files, +30 lines). Skips V dequant for near-zero attention weights during decode. No quality impact (PPL identical), 5-8%% decode speedup at 32K+ context on Metal. Happy to address any feedback.

[TheTom]

I'm not abandoning this, just parking it until #21452 is resolved.