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>

docs/evals/2026-08-03-llama-prompt-cache.md

@@ -0,0 +1,77 @@
+# Where the resident model's 7.9GB of RSS goes (2026-08-03, homesrv)
+
+Measured for Vikunja #499. The deployed llama-server held 7.9GB RSS for a 1.1GB
+model file. Half a gigabyte of it was in swap, on a box that also runs
+whisper.cpp, piper and the embedder.
+
+## Method
+
+`maven-mavend-1` was stopped for the measurement, with the owner's approval.
+Its own binary then ran on the host with the exact deployed command line. That
+binary is `/opt/maven/bin/llama-server`, version `1 (4c65955)`, a Vulkan build.
+
+```sh
+llama-server -m /mnt/hdd1/llms/qwen3/Qwen3-1.7B-UD-Q4_K_XL.gguf \
+  --host 127.0.0.1 --port 18099 -c 4096 -ngl 99 --no-webui
+```
+
+RSS was read from `/proc/<pid>/status` after load and after each of 8 distinct
+1521-token prompts. `smaps` of the deployed process was read first, from inside
+the container, since the host user cannot read another user's maps.
+
+## The cause: the prompt cache, not the weights and not the offload
+
+The startup log says it outright:
+
+```text
+srv load_model: prompt cache is enabled, size limit: 8192 MiB
+srv llama_server: n_parallel is set to auto, using n_parallel = 4 and kv_unified = true
+```
+
+The server saves the full KV state of every idle slot it evicts. It keeps up to
+8GiB of those states in host RAM (llama.cpp PR 16391). One saved prompt of 1521
+tokens costs 166.377 MiB. That is 112 kiB per token, exactly Qwen3-1.7B's KV
+footprint (28 layers x 2 x 1024 dims x 2 bytes).
+
+RSS at rest, and per distinct prompt:
+
+| Prompts served | RSS, default | RSS, `--cache-ram 512` |
+|---|---|---|
+| 0 (just loaded) | 443 MB | 411 MB |
+| 1 | 445 MB | 411 MB |
+| 4 | 958 MB | 929 MB |
+| 8 | 1641 MB | 932 MB |
+
+Uncapped, RSS climbs about 170MB per distinct prompt and does not stop until
+the 8GiB limit. Capped at 512 MiB it plateaus at 932MB from the fourth prompt
+on, with the cache holding steady at `3 prompts, 499.132 MiB` and evicting.
+
+The 7.9GB on the running daemon was that climb, weeks of it. Its `smaps` showed
+one 6.03GB anonymous mapping at 5.32GB resident plus a 1.45GB mapping at 1.27GB
+resident, and only 30MB of file-backed RSS.
+
+## The task's leading guess was wrong
+
+`-ngl 99` on the Vega iGPU costs almost no process RSS. A freshly loaded server
+has 95MB of anonymous RSS in total. RADV allocates device memory through the
+kernel, outside the process, and the log sees 8202 MiB free on `Vulkan0`. The
+weights are mmapped and file-backed, so they are evictable and do not pin RSS. The logit buffer is not visible in the numbers above at all.
+
+## Decision
+
+`--cache-ram 512` is now the default, wired as `phraser.cache_ram_mib` and set
+in `deploy/mavend.json`. 512 MiB caps total RSS near 1GB, an eighth of what the
+box carried. It still holds three of the 1521-token probes above. Maven's real
+routing and phrasing prompts are much shorter, so it holds more of those than
+the table suggests. `-c 4096` is untouched, as #499
+required. A negative `cache_ram_mib` passes no flag, for a llama-server too old
+to know it.
+
+Not changed: `n_parallel = 4`. With `kv_unified = true` the four slots share one
+4096-token KV cache, so they do not multiply it.
+
+The other half of #499 was that none of these lines were reachable. mavend
+scraped llama-server's stderr for the listen line and discarded it, and never
+piped stdout at all. Both streams now go to mavend's log with a `llama:` prefix.
+The last 12 startup lines go into the error when the server dies before it
+listens.