infra(l1): BenzoNet gas/fee sustainability + P-Chain runway guardrails

[akash-mondal]

Tracker: #154

Context

The BenzoNet testnet L1 is funded for only ~3 weeks of continuous P-Chain fees (README.md "Deployed on BenzoNet L1"); running out stops block production — the chain already halted for ~43h (#145). BGAS is a valueless gas token (drips via the Native Minter precompile, #27), so the real operational cost is the P-Chain continuous-subnet fee on the validator(s), not user gas.

Scope

• Model the P-Chain fee burn rate and required runway; define a funding cadence + a low-balance alert threshold (extends the Add monitoring, health checks, and P-Chain fee-balance alerting for BenzoNet #28 P-Chain fee-balance alerting).
• Add a runbook to top up before exhaustion and to recover from a halt (restart avalanchego block production, cf. the chore(infra): BenzoNet L1 halted — no blocks for ~43h, restart avalanchego block production #145 / Add monitoring, health checks, and P-Chain fee-balance alerting for BenzoNet #28 procedures).
• Confirm BGAS drip funding floors for onboarding/demo accounts don't independently stall flows.

Acceptance criteria

• A documented burn-rate model + minimum runway target + funding cadence.
• Low-P-Chain-balance alerting wired (or the Add monitoring, health checks, and P-Chain fee-balance alerting for BenzoNet #28 threshold confirmed sufficient) + a halt-recovery runbook.
• BGAS drip floors documented.

Dependencies

• Extends Wire BGAS gas drips via the Native Minter precompile and write the fee runbook #27 (BGAS drips / fee runbook) and Add monitoring, health checks, and P-Chain fee-balance alerting for BenzoNet #28 (monitoring + P-Chain alerting); references the chore(infra): BenzoNet L1 halted — no blocks for ~43h, restart avalanchego block production #145 halt.