feat(online): player presence replication with snapshot interpolation

[aram-devdocs]

Replicate remote players as ghost boats: position/rotation snapshots at a fixed rate with client-side interpolation and extrapolation caps. Target smooth motion at typical latency, not physics fidelity.

Blocked by #15 and #16. SnapshotCache ring buffer authored; in-world rendering is this issue.

[aram-devdocs]

Picking up the game-free half (interpolation sampler in packages/sync). Branch: feat/18-snapshot-sampler. The ghost-boat renderer (game-coupled) + in-world verification remain for a live session.

[aram-devdocs]

Reopening: PR #43 delivered only the game-free interpolation sampler (#18-A). The game-coupled ghost-boat renderer (#18-B) and in-world verification remain. (GitHub auto-closed this because the PR body contained the literal 'close #18' inside 'does not close #18' — its keyword parser ignores the negation.)

[aram-devdocs]

Handoff: #18-A (game-free interpolation sampler — Interpolator.SampleAt: LERP/SLERP, 100ms render-behind, 250ms extrapolation cap, edges + 9 tests) is MERGED to dev. Remaining = #18-B (game-coupled + live): a ghost-boat renderer under apps/Sailwind.Online.Client that spawns/moves a visible boat per remote entity, driven each frame by SampleAt; verify smooth motion in a live modded session.