feat: reduce NoteType to a 1 bit encoding and add metadata version

[PhilippGackstatter]

Summary

Reduces NoteType encoding from 2 bits to 1 bit and adds a 4-bit version field to the note metadata header, as a prerequisite for #2555 (multiple attachments per note). This frees up bits in the metadata header for future attachment metadata.

Breaking change: The note metadata encoding (both Rust serialization and MASM Word layout) has changed.

This PR also implements Default for NoteType by making NoteType::Private the default - and we could do the same for AccountStorageMode::Private when we get rid of the Network variant. This makes deriving bon::builder for notes (see #2283) even easier since we don't have to make the decision on what the default note type is for every note.

New metadata layout

Old: [sender_id_suffix (56 bits) | 6 zero bits | note_type (2 bits)]
New: [sender_id_suffix (56 bits) | reserved (3 bits) | note_type (1 bit) | version (4 bits)]

• NoteType::Private changed from 2 to 0 (new default)
• NoteType::Public remains 1
• Version is hardcoded to 0 for forward compatibility
• Reserved bits are validated to be zero on decode

[bobbinth]

Looks good! Thank you! I left a couple of small questions/comments inline.

[bobbinth]

Any reason to switch away from using SENDER_SUFFIX_MASK constant here?

[PhilippGackstatter]

Previously we had:

const NOTE_TYPE_MASK: u8 = 0b11;
const SENDER_SUFFIX_MASK: u64 = !(NOTE_TYPE_MASK as u64);

If we update the note type mask it would still technically work, but would be semantically incorrect because we'd also be extracting the version from the metadata (which happens to be 0 for now). So it seems better to explicitly mask out the least significant byte.

I brought back the constant for explicitness:

const SENDER_SUFFIX_MASK: u64 = 0xffff_ffff_ffff_ff00;