Implement PoCW consensus mechanism for ordinary transfers

[ABresting]

Summary

Ordinary transfers in the Setu consensus pipeline do not produce causally meaningful work. There is no solver competition, no quality variance, and no downstream computational impact that PoCW can evaluate. The three PoCW scoring signals (distance, necessity, contribution) degenerate when applied to transfers: any correct executor produces an identical result, so there is no meaningful basis for differentiated reward distribution. Given this, the most appropriate economic treatment for ordinary transfers is a flat fee, configurable via PoCWConfig.transfer_fee (default 21,000), that is deducted from the sender's coin alongside the transfer amount and permanently destroyed, reducing total supply. Fee enforcement occurs within the TEE execution environment: the runtime withdraws amount + fee atomically, and if the combined total exceeds available balance, the transfer produces zero state changes. This was validated at precise boundary conditions against live validator runs.

A fold-time economic observer has been introduced into the consensus pipeline to handle the reward side for solver-executed work. At each fold, the observer aggregates per-event burn and power consumption metrics, then computes per-solver PoCW scores across three DAG-derived signals: depth contribution (position of solver events within the causal chain), necessity (fraction of solver events that are ancestors of the fold's tip events), and gas usage (TEE-attested computational effort). These scores are normalized and used to distribute minted Flux proportionally to participating solvers. Cumulative rewards are persisted in the ROOT subnet's sparse Merkle tree under pocw:reward:{solver_id} keys, and each event now carries an executed_by field to distinguish the executing solver from the originating validator.

The entire economic layer is gated behind PoCWConfig.enabled. When disabled, the fee defaults to zero, no fold observer executes, no rewards are minted, and all existing behaviour remains unchanged. Configuration is read from environment variables (POCW_ENABLED, POCW_TRANSFER_FEE) via the dotenv support introduced in the companion PR #13. Naming conventions were standardised across the codebase: burn_fee was renamed to fee as the burn mechanism is an implementation detail, and transfer_fixed_fee was simplified to transfer_fee.

Closes #8

Test plan

• 15 unit tests pass: flux_burn (3), fold_observer (3), power (3), processor (6)
• Live binary: fee enforcement at exact boundary (amount + fee > balance produces 0 events processed)
• PoCW defaults to disabled; all existing tests pass with it off
• Full workspace: 105/106 pass (1 pre-existing failure unrelated to PoCW)