docs(kv): add bit-density KV cache quant table across 3 runtimes

- Place ik's Trellis (IQ*_KT), IQK (IQ*_K), MXFP4, and dedicated fast KV type
  Q8_KV in a single high->low bits-per-value table with ROCmFPX TurboQuant KV.
- Mark mainline-provided types (*), KV-native vs weight-family-reused-for-KV.
- Hadamard annotated as a transform on top of a base quant, not a row.
- bpw read from exact ggml block structs (QK_K=256, fp16 scale): type_size x 8 / QK,
  verified against the struct comment "6.5625 bits per weight" and the trait table.
  Notes where legacy nominal figures (4.0/4.25) diverge.
- AGENTS.md: add benchmark directive (use llama-bench / vllm bench; ask before
  rolling your own).

AGENTS.md

@@ -174,6 +174,9 @@ These servers serve real users (about 3 people). Running a bench while the
 service is up starves the GPU and corrupts results. Restart the service when
 the bench finishes.
 
+ALWAYS bench using llama-bench for llama.cpp or vllm bench for vllm runtimes.
+DO NOT ROLL YOUR OWN BENCH UNLESS YOU CANNOT TEST THE DESIRED PARAMETER WITH THE OFFICIAL BENCHES. ASK THE USER BEFORE DEVIATING FROM THIS DIRECTIVE.
+
 1. Stop the service before any benchmark:
    `sudo systemctl stop frank-default-small` (Dankserver) or
    `sudo systemctl stop frank-default-large` (Dankclaw).