fix(shielded): fee and transfer calculation correctness issues

[lklimek]

Summary

Several shielded transaction calculations have correctness issues that can cause failed transactions, insufficient funds errors, or premature success reporting.

Found during CodeRabbit review of PR #793 (zk branch).

⚠️ Wallet-touching files

Files marked with 🔀 touch wallet code and may conflict with the wallet refactor (PR #791 / #794). Prioritize these for early resolution.

Issues

1. Note selection ignores transaction fees (src/backend_task/shielded/bundle.rs:683)

select_notes_for_amount() selects notes to cover only the send amount, without reserving fee headroom. Transitions can fail because selected inputs don't cover amount + fees.

Fix: Add a fee_headroom: u64 parameter (or compute estimated fee internally). Select notes for amount + fee_headroom. Update all 3 callers (shielded_transfer, unshield_credits, shielded_withdrawal) to pass an estimated fee. Surface ShieldedInsufficientBalance with required = amount + fee_headroom.

2. Asset-lock fee truncated before safety buffer (src/backend_task/shielded/bundle.rs:401)

platform_fee_credits / CREDITS_PER_DUFF truncates before the 20% buffer is applied. For small amounts the truncation error is proportionally large.

Fix: Use ceiling division before applying the buffer:

let platform_fee_duffs = ((platform_fee_credits + CREDITS_PER_DUFF - 1) / CREDITS_PER_DUFF)
    .saturating_mul(120) / 100;

3. Fee retry loop uses relative scale delta (src/backend_task/core/mod.rs:678)

The loop that reduces total_amount using scale_factor stops when scale_factor change is < 0.0001. For large payments, this exits too early — the absolute duff reduction may still be significant.

Fix: Change stop condition to absolute duff delta: check whether the actual integer reduction in duffs is < MIN_DUFF_STEP (e.g., 100 duffs) or reaches zero.

4. override_fee silently ignored in SPV send path (src/backend_task/core/mod.rs:727)

The SPV send path always uses FeeRate::normal(). WalletPaymentRequest::override_fee is silently discarded.

Fix: Check request.override_fee — either convert to FeeRate and apply, or return an explicit error that override_fee is unsupported in SPV mode.

5. 🔀 Shield balance check ignores platform fees (src/ui/wallets/shield_credits_screen.rs:631)

Balance check compares amount * repeat against read_address_balance() without accounting for platform/transition fees. Users can attempt shields that will fail due to insufficient funds after fees.

Fix: Query the fee estimator, check amount * repeat + estimated_fees * repeat <= balance. Cap max amount or show error if fees make transfers unaffordable.

6. Shield reports success before nonce is usable (src/backend_task/shielded/bundle.rs:214)

shield_credits() returns Ok immediately after broadcast(). The nonce isn't updated until the next sync, so a rapid follow-up shield can reuse the old nonce and fail.

Fix: After broadcast(), call wait_for_response() (or equivalent confirmation) and bump the cached platform nonce before returning Ok(()). Mirror the batch broadcaster's follow-up logic.

Acceptance Criteria

• select_notes_for_amount accounts for estimated fees
• Asset-lock fee uses ceiling division
• Fee retry loop uses absolute duff delta
• override_fee is either applied or explicitly rejected in SPV path
• Shield balance check includes fee estimation
• Shield waits for confirmation before reporting success

_{🤖 Co-authored by Claudius the Magnificent AI Agent}

[lklimek]

Additional findings from CodeRabbit review (PR #786)

7. Batch shielding skips follow-up note sync (src/ui/wallets/shield_credits_screen.rs)

pending_refresh_task is only queued on certain success paths — batch mode can complete without triggering a note sync, leaving the balance stale until next manual refresh.

8. Done button drops queued sync task (src/ui/wallets/shield_from_asset_lock_screen.rs)

action starts as AppAction::BackendTask(sync_task) when a pending refresh exists, but clicking Done overwrites it with AppAction::PopScreen, silently dropping the sync.

9. Max amount ignores asset-lock fee (src/ui/wallets/shield_from_asset_lock_screen.rs)

max_credits is derived from full Core balance, but shield_from_asset_lock deducts platform fees internally. User sees "max" but the actual shield amount is less.

10. Done button drops queued sync in unshield screen (src/ui/wallets/unshield_credits_screen.rs)

Same pattern as #8 — Done overwrites the queued SyncNotes action.

11. Max amount ignores transition fees in unshield (src/ui/wallets/unshield_credits_screen.rs)

set_max_amount(Some(self.max_balance)) presents the entire shielded balance, but the transition requires fees. User attempts max → fails.

_{🤖 Co-authored by Claudius the Magnificent AI Agent}