Class ContextPressure

Immutable KV budget snapshot for one tick of the agent loop

Created from SessionContext._storeKvPressure() which returns { nCtx, cellsUsed, remaining } where remaining = nCtx - cellsUsed. cellsUsed tracks unique KV cells per branch — incremented on decode_each / decode_scatter, decremented on release by position - fork_head (unique cells above the fork point), reset on bulk ops like retainOnly and drain.

Two thresholds partition remaining into three zones:

┌──────────────────────────────────────────────────────┐
│                    nCtx                              │
│  ┌──────────┬───────────────────┬──────────────────┐ │
│  │cellsUsed │    headroom > 0   │    softLimit     │ │
│  │ (in use) │   (new work OK)   │   (reserved)     │ │
│  └──────────┴───────────────────┴──────────────────┘ │
│              ◄── remaining ──►  │                    │
│                                 │                    │
│  headroom = remaining - softLimit                    │
│  critical = remaining < hardLimit                    │
└──────────────────────────────────────────────────────┘
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• headroom > 0 — room for new work (tool results, generation)
• headroom ≤ 0 — over budget. SETTLE rejects tool results, PRODUCE hard-cuts non-terminal tool calls. Terminal tools still pass.
• critical — remaining below hardLimit. Agents killed before produceSync() to prevent llama_decode crashes.