feat(save): add the atomic field-save engine and the assignee saver

pearl--run-atomic-field-save is the structured-field sibling of the text engine pearl--run-field-save. It runs the atomic three-way over a spec's live and baseline values. A missing baseline skips with missing-property (a legacy file — refresh first). Live equal to baseline is unchanged. Otherwise it fetches the remote and compares: remote already equal to live converges and advances the baseline, remote unmoved from baseline is a clean push, and a genuine divergence runs the atomic conflict gate. It reuses the existing save-outcome contract, so structured fields report the same status/reason shape as title and description.

pearl--save-assignee-field is the first saver on it — the cleanest relation field. Its live id is LINEAR-ASSIGNEE-ID diffed against LINEAR-ASSIGNEE-ID-SYNCED, the remote comes from the issue node, the push sends assigneeId, and use-remote rewrites the display name from the fetched node. Priority, state, and labels follow the same template in the next commit.

It's not wired into save-issue yet — that's the next commit. I tested it in isolation: the engine's three-way and conflict branches over a synthetic spec, plus the assignee saver end to end with a stubbed fetch and update.

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