bip21_uri

• fix: amount was encoded through double.toString(), which switches to scientific notation below 1e-6 — one satoshi was emitted as amount=1e-8, which no BIP21 grammar accepts (an optional X/x exponent, never e). Amounts are now formatted at satoshi precision with trailing zeros trimmed, so canonical values such as 20.3 are unchanged.
• fix: the pj fragment separator is preserved as received instead of being rewritten to +. - is the separator of the current payjoin specification and + the legacy one; rust-payjoin accepts both and chooses between them in check_fragment_delimiter, so the 2.1.2 retrocompatibility shim for payjoin-flutter now only turns current URIs back into their deprecated form. Behaviour change: decoding a - separated endpoint no longer returns a + separated one.
• fix: only the BIP77 fragment is uppercased. The URL before it is case-sensitive, so a directory host containing a hyphen, or a mailbox path that is not already uppercase, is no longer corrupted. A BIP78 v1 endpoint, which has no fragment, is left untouched.

• fix: illegal percent encoding

• refactor: payjoin-flutter is not compatible with hyphens - symbol defined in the latest payjoin specification. To ensure retrocompatibility with payjoin-flutter, bip21_uri is returning the pj field with + sign.

• fix: regression on uppercase

• If pj is passed among the options of the query parameters, the package now process it as a payjoin Uri specified in BIP77 supporting both percent encoded or not, + and -. Keep the # encoded as %23 and the Uri is uppercase.
• Payjoin unit tests

• Empty amount is decoded as null
• Empty message or label is decoded/encoded as empty
• Empty options are encoded/decoded

Breaking Changes

• Package renamed: dart_bip21 → bip21_uri
• Model renamed: BIP21 → Bip21Uri
• Amount type changed: num? → double?
• API restructured: Complete rewrite of the codebase with new file structure