feat(v2): principal-based fee split — fixes super under-payment (#38 3/5)

Replaces the broken fraction-of-fee math with fraction-of-principal,
direction-aware. Pre-#38: super_fee_fraction was interpreted as
`round(fee_sats * super_fraction)`, paying super ~13× below intent on
every cashout since the bitspire wire-shape landed. Post-#38: super
and operator shares are computed independently against principal
using the per-direction fractions from SuperConfig + Machine.

Per workspace CLAUDE.md "Backwards-compatibility on pre-public-launch
code" (v2-bitspire hasn't shipped to users), no compat shims:

- calculations.py: delete `split_two_stage_commission` (legacy
  fraction-of-fee). Keep `split_principal_based` as the sole split fn.
- migrations.py m009: extend to also DROP the deprecated
  `super_fee_fraction` column after backfilling its value into the
  new directional fields.
- models.py: drop `super_fee_fraction` from SuperConfig +
  UpdateSuperConfigData entirely.
- bitspire.py parse_settlement: new signature takes `super_config:
  SuperConfig` instead of `super_fee_fraction: float`. Resolves
  directional fractions from super_config + machine by tx_type, then
  computes via split_principal_based. Raises SettlementInvariantError
  on unknown tx_type.
- tasks.py: pass `super_config` through to parse_settlement; assert
  non-None (m001 inserts the singleton at install time — None is an
  impossible state).
- partial-dispense ratio path in distribution.py is unchanged — still
  uses `settlement.platform_fee_sats / settlement.fee_sats` from the
  landed row, which is the right invariant (lock at landing) and
  independent of the per-direction config.

Tests:
- Rename `test_two_stage_split.py` → `test_operator_split_legs.py`.
  Drop the legacy-function test classes. Keep TestAllocateOperatorSplitLegs
  (still-production fn) and TestPartialDispenseSplitRatio (inline ratio
  math in distribution.py).
- New `test_principal_based_fees.py`: pure-math tests for
  `split_principal_based` (six cases including a direct regression
  test pinning the pre-#38 bug at 240→3000 sats per 100k principal at
  3% super), plus parse_settlement directional dispatch tests
  (cash-in routes through cash-in fractions; cash-out through
  cash-out; unknown tx_type raises; zero-zero free-charge ATM; cross-
  direction guard).

Migration verified end-to-end via container restart: super_config
columns post-m009 = id/super_fee_wallet_id/updated_at/
super_cash_in_fee_fraction/super_cash_out_fee_fraction (no
super_fee_fraction). dca_machines + dca_settlements gained the
expected new columns. 156/156 tests green.

Refs: aiolabs/satmachineadmin#37 (parent), #38 (this layer). Closes
the load-bearing super under-payment bug standalone.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tasks.py

@@ -125,14 +125,14 @@ async def _handle_payment(payment: Payment) -> None:
     # stamp is missing, SettlementInvariantError on any range/sum
     # breach.
     super_config = await get_super_config()
-    super_fee_fraction = float(super_config.super_fee_fraction) if super_config else 0.0
+    assert super_config is not None  # m001 inserts the default singleton
     try:
         data = parse_settlement(
             machine=machine,
             payment_hash=payment.payment_hash,
             wire_sats=payment.sat,
             extra=extra,
-            super_fee_fraction=super_fee_fraction,
+            super_config=super_config,
         )
     except (SettlementMetadataError, SettlementInvariantError) as exc:
         await _record_rejected(payment, machine, exc)