Demand-based model wants: unified gossiped demand map

[michaelneale]

Problem

When a node declares --model GLM and later leaves the mesh, the "want" for GLM disappears entirely. The mesh forgets it ever wanted GLM. New nodes joining see no demand and go standby, even if they have GLM on disk and spare VRAM.

This happened because model wants were tracked in three disconnected ways:

• requested_models — per-node, gossiped, dies when the node leaves
• mesh_wanted — local in-memory accumulator, never gossiped, dies on restart
• request_rates — per-node request counts, not used for want discovery

Solution

Replace all three with a single model_demand map that gets gossiped infectiously across the mesh:

pub struct ModelDemand {
    pub last_active: u64,      // unix timestamp of last request or declaration
    pub request_count: u64,    // total requests seen (merged via max)
}

How it works

• --model flags seed demand entries at startup
• API requests (including misses/503s for unserved models) refresh last_active and increment request_count
• Every gossip exchange carries the full mesh-wide demand map — it propagates infectiously
• Merge via max: last_active = max(ours, theirs), request_count = max(ours, theirs)
• Natural decay: entries older than 2 hours (DEMAND_TTL_SECS) are ignored unless pinned by a live --model node
• pick_model_assignment() prioritizes by demand (hottest unserved model first)

What this fixes

1. Node leaves, want survives — demand was gossiped to all nodes, they keep spreading it
2. API request for unserved model creates demand — someone asks for Qwen3-8B, it enters the demand map, standby nodes can pick it up
3. One concept instead of three — model_demand replaces requested_models + mesh_wanted + request_rates
4. Nostr wanted list derived from active demand instead of per-node requested_models

Backward compatible

• Old nodes ignore the new model_demand field (serde skips unknown fields)
• New nodes receiving from old nodes synthesize demand entries from requested_models + request_rates
• requested_models and request_rates still gossiped for old node compat

Tests

7 new unit tests covering merge logic, TTL filtering, backward compat synthesis, and serialization.