Remove dead members from Snapshot and SyncPlan

[in-jun]

The core-sync models carry four public members with no call sites anywhere in the repo. A small, provable model surface is the point of core-sync, so these are worth dropping before someone reaches for one and introduces an inconsistent code path.

• Snapshot.with/Snapshot.without: mutation helpers on an otherwise-immutable model; snapshots are always rebuilt in full via SnapshotBuilder, never mutated incrementally.
• SyncPlan.hasConflicts and SyncPlan.EMPTY: unused; conflict reporting flows through the differently-named SyncResult.Success.hadConflicts. Removing EMPTY leaves an empty companion object, so that goes too.

Kept Snapshot.EMPTY/of/contains and SyncPlan.conflicts/localDeletionCount/remoteDeletionCount/action, which are all still referenced. core-sync compiles and its full test suite passes.

Fixes #73

[in-jun]

Verified against the current tree: a repo-wide search for .with(, .without(, hasConflicts, and SyncPlan.EMPTY (Kotlin sources plus build scripts) turns up only the declarations being removed here. Conflict reporting genuinely flows through the separately-named SyncResult.Success.hadConflicts, and every surviving .EMPTY reference in the tests resolves to Snapshot.EMPTY, which is retained. Dropping the now-empty SyncPlan companion object along with EMPTY is the right call.

Scope is tight — nothing beyond the four dead members — and it lands squarely on the stated goal of keeping the core-sync model surface small and provable. Looks good to merge.