fix(epoch): emit decayed old bond entries absent from new in mat_ema_alpha_sparse

[hconsulting987654321-blip]

Summary

• mat_ema_alpha_sparse only emitted sparse output entries present in the new matrix. Old bond entries absent from new_row had their decayed values computed ((1 - alpha) * old_val) but never emitted, causing bonds to drop to zero instantly instead of decaying gradually.
• This diverges from the dense equivalent mat_ema_alpha and breaks the bond decay semantics: when a validator removes weight to a miner, the bond should decay by (1 - alpha) per epoch, not zero immediately.
• Fix: after the new_row loop, iterate decayed_values for columns not in new_row and emit non-zero entries. Output is sorted by column index to maintain the sparse row invariant.
• Updated test_math_sparse_mat_ema_alpha to assert the correct value (0.9 instead of 0.0 for alpha=0.1, old=1.0, new=0.0).

Root cause

// Before: only iterates new_row, missing old-only entries
for &(j, new_val) in new_row.iter() {
    // emit alpha*new + (1-alpha)*old for columns in new
}
// old-only columns → decayed_values computed but never pushed to out_row

Fix

// After: also emit decayed old entries for columns absent from new_row
let mut new_cols = std::collections::BTreeSet::new();
for &(j, new_val) in new_row.iter() {
    new_cols.insert(j);
    // ... existing logic ...
}
for (j, &decayed) in decayed_values.iter().enumerate() {
    if !new_cols.contains(&(j as u16)) && decayed > zero {
        out_row.push((j as u16, decayed));
    }
}
out_row.sort_unstable_by_key(|&(j, _)| j);

Test plan

• cargo test -p pallet-subtensor -- mat_ema — all 15 tests pass
• test_math_sparse_mat_ema_alpha now correctly validates old-only-entry decay
• All existing sparse EMA tests (test_mat_ema_alpha_sparse_*) continue to pass

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