Epic: #268 | Phase: 4 — Types & API | Priority: P1-P2

R6.1 — Newtype for ConversationId/MessageId [P1, S]

Replace bare i64 with ConversationId(i64) and MessageId(i64) in zeph-memory/zeph-core.

R6.2 — Newtype for Embedding [P2, S]

Embedding(Vec<f32>) instead of raw Vec<f32>.

R6.3 — Replace bool params with enums [P2, S]

is_summary: bool → MessageKind { Regular, Summary }.

R7.1 — pub(crate) audit [P1, M]

Audit all pub fn in non-lib.rs files. Restrict to pub(crate) where possible.

R7.2 — Re-export hygiene [P2, S]

Add pub use re-exports in zeph-core/src/lib.rs for key types.