[1/4] buzz-sdk: own the NIP-AP persona and team event shapes

[Maxwellimus]

Buzz Desktop was the only implementation of the kind:30175 / kind:30176 wire
format. Anything else that wanted to publish an agent definition had to
re-derive it, and a re-derivation that gets serde field order wrong produces
different content bytes, a different event id, and a persona Desktop treats as
unrelated to the one it already has.

Moves the builders, content structs, and coordinate helpers into
buzz_sdk::agent_definitions; Desktop calls them instead of holding its own
copy. double_option moves with them, because the persona/team tri-state
(absent = "unknown, preserve local" vs. null/[] = "explicitly cleared") is
part of the wire contract rather than a Desktop detail.

Field order is pinned by a byte-exact serialization test — a reorder would
silently invalidate every stored persona_content_hash.

No behavior change; this is the extraction the rest of the stack builds on.

Test plan

• just ci
• cargo test --manifest-path desktop/src-tauri/Cargo.toml — existing persona
  and team event tests pass unchanged against the moved code.

[wolfyy970]

Reviewed at exact head 1341b815ac4d. Moving the event shapes, tri-state team fields, coordinate helpers, and byte-stable persona serialization into buzz-sdk is the right first increment. Desktop still projects only explicit shared fields, and the extraction does not move launch, credentials, or machine configuration into the definition.

I reran the SDK contract tests plus Desktop persona/team event suites: 58 passed. Both Rust format checks and git diff --check are clean. The red smoke check is an unrelated Huddle voice-menu failure.