License inconsistencies across workspace crates

[spacebear21]

Surfaced while preparing the Dart binding for pub.dev publication.

The workspace declares three different licenses, with a likely-incorrect
Apache-2.0 grant on payjoin-ffi:

Crate	Cargo.toml license	LICENSE files committed
payjoin	MITNFA	none
payjoin-cli	MITNFA	none
payjoin-mailroom	MITNFA	none
payjoin-test-utils	MIT	none
payjoin-ffi	MIT OR Apache-2.0	LICENSE.md, LICENSE-APACHE, LICENSE-MIT
payjoin-ffi/dart/native	MIT OR Apache-2.0	(inherits parent)

Issues

1. payjoin-ffi cannot relicense upstream as Apache-2.0. It depends on
   payjoin, which is MITNFA. The compiled libpayjoin_ffi statically
   embeds payjoin source. Apache-2.0 introduces grants and obligations
   (notably the patent grant and stricter notice requirements) that downstream
   consumers cannot rely on, because the embedded MITNFA portions don't carry
   those grants. The "OR Apache-2.0" promise is misleading.

2. payjoin-test-utils is plain MIT rather than MITNFA, dropping the
   no-false-attribs clause that the rest of the workspace carries. Probably an oversight.

3. The MITNFA crates ship no LICENSE text. Only the SPDX identifier is
   declared in Cargo.toml. crates.io accepts this, but consumers outside
   crates.io (e.g. distros, SBOM scanners) get no canonical text.

Suggested resolution path

1. Relicense payjoin-ffi to MITNFA and remove LICENSE-APACHE. This is fine because the "OR Apache-2.0" part is being overriden by the compiled payjoin's more restrictive MITNFA already.
2. Relicense payjoin-test-utils to MITNFA.
3. Add canonical MITNFA text to a LICENSE file in the top-level repo (and/or individual crates?).

Disclaimer: IANAL and Claude surfaced and wrote this. It all seems reasonable to me though.

[nothingmuch]

I am signing off on relicensing my good contributions under MITNFA. I must however insist that any bad code that I added to payjoin-ffi remains licensed under Apache-2.0, so unfortunately that will all have to be replaced.