Add Qwen2.5-Coder-14B-Instruct-4bit to astro-orbiter router (t_55c164f5)

- host_vars/astro-orbiter/vars.yml: add Qwen2.5-Coder-14B-Instruct-Q4_K_M.gguf
  to llm_staged_models (size_bytes=8988111072, bartowski GGUF public repo).
  Updated VRAM note to reflect 4-model roster and LRU eviction semantics.
- defaults/main.yml: add llm_router_coder_ctx_size=16384 and
  llm_router_coder_flash_attn=true variables for per-model ctx tuning.
- templates/llama-server-router-preset.ini.j2: add [Qwen2.5-Coder-14B-Instruct-Q4_K_M]
  section with alias=Qwen2.5-Coder-14B-Instruct-4bit, ctx-size=16384, flash-attn=true.
- playbooks/day2_add_coder_alias.yml: new playbook that downloads the GGUF (if
  absent/mismatched), deploys updated preset INI and systemd unit, restarts
  llama-server-router, and verifies all 4 models in /v1/models.

VRAM: Coder ~9GB. Full 4-model co-residency impossible on 24GB — LRU eviction
handles this automatically. Qwen3.6-35B <-> Coder switches incur ~30-60s cold load.

ansible/host_vars/astro-orbiter/vars.yml

@@ -34,20 +34,25 @@ common_root_lv: ubuntu-lv
 # (t_33acbb2e) so the router can keep more than one GGUF resident on-demand
 # and LRU-evict when needed.
 #
-# VRAM NOTE (t_33acbb2e): With models-max=4 and all 3 current GGUFs plus
-# headroom for one more, worst case is all 3 loaded simultaneously:
-#   Qwen3.6-35B-A3B Q4_K_S:       ~21.5GB (weights ~19.5GB + KV ~2GB @ 64K ctx, q4_0)
-#   Phi-3.5-mini-instruct Q8_0:   ~4.3GB  (weights ~3.8GB + KV ~0.5GB @ 64K ctx)
-#   Meta-Llama-3.1-8B Q4_K_M:    ~5.6GB  (weights ~4.6GB + KV ~1.0GB @ 64K ctx)
-#   Total worst-case:             ~31.4GB  >  24GB RTX 3090
+# VRAM NOTE (t_33acbb2e, updated t_55c164f5): With models-max=4 and all 4 GGUFs
+# registered, worst case is all 4 loaded simultaneously:
+#   Qwen3.6-35B-A3B Q4_K_S:           ~21.5GB (weights ~19.5GB + KV ~2GB @ 64K ctx, q4_0)
+#   Phi-3.5-mini-instruct Q8_0:       ~4.3GB  (weights ~3.8GB + KV ~0.5GB @ 32K ctx)
+#   Meta-Llama-3.1-8B Q4_K_M:        ~5.6GB  (weights ~4.6GB + KV ~0.2GB @ 8K ctx)
+#   Qwen2.5-Coder-14B Q4_K_M:        ~9.0GB  (weights ~8.4GB + KV ~0.6GB @ 16K ctx)
+#   Total worst-case:                 ~40.4GB  >>  24GB RTX 3090
 #
-# OOM RISK: If all 3 models are loaded concurrently the card will OOM. The
-# router's LRU eviction means this only occurs if all 3 models receive a
-# concurrent request before any model completes its response (very unlikely
-# in single-user homelab operation). However, Ryan should be aware. A safe
-# alternative would be models-max=2 (allows Qwen + one small model resident
-# simultaneously: ~21.5 + 5.6 = ~27.1GB still tight). Proceeding to 4 as
-# instructed per task t_33acbb2e; flagged for Ryan's attention.
+# OOM RISK: Full co-residency is impossible on 24GB. LRU eviction prevents this
+# in practice: models-max=4 means the router can REGISTER 4 models but only keeps
+# up to 4 LOADED simultaneously — the router will evict the LRU model when a new
+# one is needed. In single-user homelab operation, only one model is active at a
+# time. The realistic maximum co-residency is 2 models (whichever was last used).
+# Qwen3.6-35B alone uses ~21.5GB; co-residency with Coder (~9GB) = ~30.5GB > 24GB.
+# So effectively: Qwen3.6-35B + any second model will OOM IF both are held concurrently.
+# LRU eviction handles this automatically — the router evicts the idle model before
+# loading the new one. Ryan should be aware this means model-switching always incurs
+# a ~30-60s cold-load latency when switching between Qwen3.6-35B and any other model.
+# Proceeding to models-max=4 as instructed; flagged for Ryan's attention.
 llm_router_models_max: 4
 
 llm_staged_models:
@@ -59,4 +64,8 @@ llm_staged_models:
     url: "https://huggingface.co/bartowski/Meta-Llama-3.1-8B-Instruct-GGUF/resolve/main/Meta-Llama-3.1-8B-Instruct-Q4_K_M.gguf"
     size_bytes: 4920739232
     source_repo: "bartowski/Meta-Llama-3.1-8B-Instruct-GGUF"
+  - filename: "Qwen2.5-Coder-14B-Instruct-Q4_K_M.gguf"
+    url: "https://huggingface.co/bartowski/Qwen2.5-Coder-14B-Instruct-GGUF/resolve/main/Qwen2.5-Coder-14B-Instruct-Q4_K_M.gguf"
+    size_bytes: 8988111072
+    source_repo: "bartowski/Qwen2.5-Coder-14B-Instruct-GGUF"