feat: allowances

[ovitrif]

Twin: synonymdev/bitkit-ios#805
Refs:

• Validate immutable Payment Request domain values pubky/paykit-rs#158
• Add shared Allowance matching and limit evaluation pubky/paykit-rs#159
• Implement durable Allowance payment accounting pubky/paykit-rs#160
• Expose Allowance accounting to Swift and Kotlin pubky/paykit-rs#161

Problem or use case

A Paykit contact who sends small, frequent payment requests, such as a creator's paid posts or a shop's small orders, makes the payer approve each one by hand. The payer has no way to say "pay this contact's small requests without asking me".

Proposed solution

Bitkit supports Paykit allowances: the payer (the allower) sets a per-payment limit and a monthly limit for one contact (the allowee), and that contact's requests within both limits are paid without a prompt.

Done when:

• An allower can set, view and end an allowance for a contact, and the allowee sees it as active without extra steps.
• A request within both limits is paid automatically and never twice, including across an app kill or restart.
• These requests are handled as ordinary Payment Requests:
  • above the per-payment limit
  • over the monthly cap
  • after either side ends the allowance
• The allower is told when a limit stopped an automatic payment.
• The allower learns about waiting requests even when Bitkit is not open.

Alternatives considered

A local auto-approve rule on the payer's wallet only. It needs no protocol, but the allowee cannot see or end the allowance, cannot ask for one, and payment proofs cannot name the allowance.

Additional context

• Protocol: specs/allowances.md in pubky/paykit-rs.
• The SDK and Swift/Kotlin bindings in the refs above are not merged yet. Bitkit builds against a local build of them until they are published.
• Design, Figma "Bitkit - Experimental New":
  • Allowances empty state
  • Set Allowance
  • Allowances list