Implement merge from its specification

merge in src/notesapi/sync.py is declared, documented and tested, but its

body raises NotImplementedError. Write it.

The full contract — what counts as a write, how a higher updated_at wins, how

tombstones behave, and what the result object must expose — is in the module's

docstring. Read it before writing a line of code; it is the specification, and

the evaluation measures against it.

The short version

• A note the client has and the server does not is an insert.
• A note both have, where the client's updated_at is strictly newer, is an

update; when the note is a tombstone, the update is a delete.

• A note both have with the same updated_at: identical bodies and tags is

unchanged, anything else is a conflict — the merge does not guess.

• A note the server has and the client does not is left alone; the client copy

is not a list of everything that exists.

• The result's wins holds the notes the server should now carry, and the five

lists of ids never overlap.

Boundaries

• Implement it in src/notesapi/sync.py; the public API of notesapi stays

as it is.

• Do not modify the repository test suite under tests/ and do not add

third-party dependencies — the standard library is enough.

• The repository's own test suite (pytest) has to stay green, so the

store, errors and search modules and their behaviour must keep working.