phraser: cap llama-server's prompt cache at 512 MiB (V-499)

The forwarded log named the cause in one line: the prompt cache limit
defaults to 8192 MiB. llama-server saves the full KV state of every idle
slot it evicts, 112 kiB per token, so RSS climbed about 170MB per
distinct prompt until the deployed server held 7.9GB for a 1.1GB model.

Measured on homesrv today, uncapped versus `--cache-ram 512`: RSS
plateaus at 932MB from the fourth distinct prompt instead of climbing.
The task's leading guess was wrong. `-ngl 99` costs almost no RSS,
because RADV keeps device memory outside the process. Numbers and method
in docs/evals/2026-08-03-llama-prompt-cache.md.

`-c 4096` is untouched. The knob is `phraser.cache_ram_mib`, unset means
512, negative passes no flag for a llama-server too old to know it.

The deploy still runs the old image, so the box keeps its 8 GiB default
until mavend is rebuilt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/mavend.json

@@ -20,6 +20,7 @@
     "bin_path": "llama-server",
     "n_gpu_layers": 99,
     "n_ctx": 4096,
+    "cache_ram_mib": 512,
     "timeout": "60s",
     "llm_nudges": false
   },