spec(glue): settlement allocation follows the payment, not only its arrival

[delchev]

The settlements section described the shape but left the when implicit, and every implementation read it as "once, on arrival". A payment is rarely final when it is typed in: one booked for the wrong amount is corrected the next day, one entered from a bank statement is completed once the counter-party is identified. The allocation - and with it every invoice's settled figure - was left describing an amount the payment no longer carried.

Specifies allocation over the payment's whole life:

• it runs on the payment's create and on every subsequent change;
• each run allocates the payment's unallocated balance (pot less what is already allocated through the junction) rather than the pot again, so a re-delivered or replayed change leaves the allocation unchanged;
• a negative balance - the payment now covers less than it is allocated to - releases the excess, newest allocation first, reducing rather than removing the last allocation it touches;
• releases are ordinary junction writes, so the invoice paid roll-up follows them down.

Companion proposal: IntentFile/intent-specification#44. Implementation: eclipse-dirigible/dirigible#6818.

npx vitepress build docs passes.

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