fix: sum emission across mechanisms instead of weighting it

[kyron1112567]

Fixes #2194.

Problem

epoch_with_mechanisms merges each mechanism's EpochTerms, but aggregates two dimensionally different kinds of field the same way:

acc_terms.validator_emission = acc_terms.validator_emission.saturating_add(terms.validator_emission);
acc_terms.server_emission    = acc_terms.server_emission.saturating_add(terms.server_emission);
...
acc_terms.emission = Self::weighted_acc_alpha(acc_terms.emission, terms.emission, sub_weight);

The fields weighted alongside emission — consensus, dividend, validator_trust, stake_weight — are normalized ratios, where a weighted sum is correct. emission is an absolute alpha amount: per mechanism it equals server_emission + validator_emission, the two fields summed immediately above it. Weighting it reports a fraction of what the hotkey was actually paid.

persist_netuid_epoch_terms writes that value into Emission, which is what the metagraph reports per UID. On an even N-mechanism split it reads ~1/N of the real figure.

This was first reported in #2194 (Nov 2025) and independently confirmed by the owner of a subnet running mechanisms on mainnet. The issue was closed on 2026-07-31 with no linked commit or PR; the defect is unchanged on main.

Not just display

get_neuron_to_prune reads Emission and selects the lowest value. On an uneven split the weighting can invert relative order between mechanism-specialized neurons:

Subnet emits 1000, split 25/75. Neuron A earns 100 in mech0, neuron B earns 50 in mech1.
Actual: A=100, B=50 — B is the lower earner.
Reported: A=25, B=37.5 — A is pruned instead.

So on multi-mechanism subnets with a non-even split this can deregister the higher earner. That is a behaviour change, hence the spec_version bump.

Payout-neutral

epoch_with_mechanisms returns (hotkey, terms.server_emission, terms.validator_emission) and never returns terms.emission. Those two were already summed and are untouched here, so no hotkey's alpha changes. This matches @dougsillars' original observation that miners were being paid correctly and only the reported value was wrong.

Change

• Sum emission across mechanisms instead of weighting it, in both the and_modify and or_insert_with arms.
• Drop weighted_acc_alpha, now unused.
• spec_version 442 → 443.

Tests

Added epoch_with_mechanisms_persisted_emission_matches_paid_emission: asserts persisted Emission per UID equals server_emission + validator_emission for that hotkey, and that the per-UID total accounts for the whole epoch emission. It uses an uneven 25/75 split with each miner favoured by a different mechanism, so the weighting cannot cancel out.

Against the unfixed aggregation it fails as expected:

assert_abs_diff_eq!(reported, credited, epsilon = 8)
    left  = 312505722   // Emission storage
    right = 500000000   // server_emission + validator_emission

Also updated the emission assertion in epoch_with_mechanisms_persists_and_aggregates_all_terms, which encoded the previous behaviour.

Locally: cargo fmt --check --all clean, cargo clippy -p pallet-subtensor --all-targets -- -D warnings clean, cargo test -p pallet-subtensor --lib 1397 passed / 0 failed.

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