Generalize the atomic schema-mutation fix to field insert/reorder

[brylie]

`FieldMenu.svelte`'s `insertField` (insert left/right) and `FieldManagerDialog.svelte`'s `moveField` (reorder) both build a whole replacement schema array from a caller-supplied reactive `schema` prop and call `updateCollectionSchema` directly. Two rapid schema edits — a reorder racing an insert, two reorders in quick succession, or an edit racing a concurrent collaborator's own field change — can each read a schema that's already stale by the time the second call writes, silently dropping the first change.

PR #200 (issue #189) fixed exactly this race for field-append by adding `records.ts`'s `appendCollectionField`, which reads `ymeta.get('schema')` fresh inside a `doc.transact` rather than trusting a snapshot (flagged by CodeRabbit during that PR's review). `insertField` and `moveField` still have the old, racy shape.

• Add `records.ts` primitives for insert-at-index and reorder that read the current schema from Yjs inside `doc.transact`, mirroring `appendCollectionField`.
• Repoint `FieldMenu.svelte`'s `insertField` and `FieldManagerDialog.svelte`'s `moveField` at the new primitives, dropping the schema-snapshot parameter the same way `appendCollectionField`'s callers already did.
• Add a regression test for two rapid operations (e.g. a reorder racing an insert) from one initial schema snapshot, mirroring `appendCollectionField`'s test in `records.test.ts`.

Done when: field insert and reorder both read the schema atomically from Yjs at write time, proven by a regression test that two rapid operations from one snapshot don't drop either change.

[brylie]

Set Priority P1 (unset → P1): same data-loss-race class CodeRabbit flagged as 🟠 Major during PR #200's review (the identical bug, already fixed for field-append), just in two sibling call sites that were out of that PR's scope. Concurrent multi-actor Collection editing is this app's core value proposition, so a race that silently drops a field edit under normal collaborative use warrants prompt attention — not urgent enough to block anything, but higher than routine polish.