feat(serve): set APEX I-Compact as default, harden benchmark workflow

Serving:
- make serve now launches Claude-distilled APEX 35B-A3B (16GB) with 2
  parallel slots and 256K context as the daily driver
- add serve-custom for ad-hoc model testing
- add flush-gpu to reclaim unified memory after stuck runs

Benchmarks:
- default Vulkan-only backends (ROCm trails at long context)
- add --backends filter to run-baseline.sh
- fix backend filter substring bug (grep -qFx for exact line match)
- fix model filter regex metacharacter bug (grep -qiF for literal)
- respect --tg in long-context tests instead of hardcoded n=32

ROCm bump to 7.2.1 (kernel 6.18.4+ patch); keep 7.2 as optional.

Catalog:
- add mudler APEX I-Compact (Claude-distilled 35B, 17GB)
- add 0xSero REAP-40 (pruned 122B-A10B, 46GB)
- update download instructions: hf download (huggingface-cli is gone)

scripts/serve/launch.sh

@@ -79,6 +79,7 @@ declare -A SERVER_PATHS=(
     [llama-vulkan-amdvlk]="/usr/sbin/llama-server"
     [llama-rocm-6.4.4]="/usr/local/bin/llama-server"
     [llama-rocm-7.2]="/usr/local/bin/llama-server"
+    [llama-rocm-7.2.1]="/usr/local/bin/llama-server"
     [llama-rocm7-nightlies]="/usr/local/bin/llama-server"
 )