`_cashdro_payment_url()` is shared by the payment and the reversal paths, so
patching it applied the rounding to the reversals too: giving back a paid
order sent the drawer the *order* remaining, rounded, instead of the amount
of the line, and reversing an order already covered by other payments sent 0.
It also collided with the modules that fix that same method, leaving the
outcome to the asset load order.
Both go away by hooking `_cashdro_amount()`, the extension point
`pos_payment_method_cashdro_flow_fix` provides, which is only reached on a
payment. `shouldRound`/`getDefaultAmountDueToPayIn` came with the 18.0 tax
totals rework, so fall back instead of breaking the payment with a TypeError
the operator would read as one more connection error.
Adds the README_DEV.md the repo conventions ask for.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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