Move the Default Payjoin FFI API to primitives

[chavic]

Following up on rust-payjoin#738 to make the FFI boundary primitive-first. Strong types at the edge forced downstreams to depend on bitcoin_ffi and broke whenever wrappers drifted. We now expose strings/bytes/integers with runtime validation.

Changes

• ReceiverBuilder::new takes an address string; errors surface as ReceiverBuilderError/AddressParseError. Added PlainTxIn/TxOut/OutPoint/PsbtInput/Weight plus InputPairError for the rest of the receiver typestates (script callbacks, output substitution, input contribution, PSBT parsing).
• Sender alignment: Mirrored the upstream FSM (ProposalReceived is gone) and added helpers on SenderSessionOutcome to expose success/failure/cancel plus base64 PSBT.
• PSBT handoff: ProvisionalProposal::psbt_to_sign now returns base64, so wallets can consume it without wrappers. Fee-rate setters validate raw u64 and report overflow.
• bitcoin_ffi removed: Deleted the module/exports and cleaned constants; uniffi is bumped to 0.30 with refreshed lockfiles.
• Bindings/test harnesses updated: Dart/JS/Python harnesses use the new primitives and validation paths; regenerated bindings accordingly.

Notes

• Runtime validation remains in the core; conversions happen at the edge to avoid version lockstep on wrappers.

[DanGould]

Thanks for sharing your draft in public early before it's ready for review

[coveralls]

Pull Request Test Coverage Report for Build 20280641768

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.402%

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Totals
Change from base Build 20276641686:	0.0%
Covered Lines:	9728
Relevant Lines:	11664

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💛 - Coveralls

[chavic]

Ready for review @spacebear21 @DanGould

[arminsabouri]

Assigned @spacebear21 to this one
@chavic two things

• This PR needs a rebase as there are conflicts
• Please consider squashing some of your commits per the contributing.md guide

[spacebear21]

Thanks Armin - I agree this will be much easier to review once it's rebased and commits are organized.

[chavic]

will do

[chavic]

@arminsabouri and @spacebear21 squashed much of the git history to make things much cleaner

[benalleng]

Everything looks pretty good just the small nit about the uniffi-downgrade and upgrade one after the other.

I was able to test python but couldn't build dart or javascript properly to test

[spacebear21]

Looking good! I also think the three "adapt harness" commits could be squashed into one since they kind of all do the same thing.

I think the last two commits may also be squashed, which would resolve @benalleng 's comment.

[benalleng]

Ack 3187533 those are some clean commit messages 👀

[spacebear21]

ACK 3187533, great work on the rebase and commit organization!