refactor(undo): promote snapshots from array shapes to value classes

[oxyc]

Last week's level-6 PHPDoc burn-down gave Snapshot::postFields() etc. a typed array<string, mixed> return. Plain shapes leave half the value on the table:

• bugs only surface at first use ($snap['nme'] typo passes PHPStan, blows up at runtime)
• the shape lives in PHPDoc only — easy to lie about
• consumers re-state the field list at every call site

This PR is the direct follow-up: promote the five snapshot kinds to small value classes under src/Undo/Snapshots/. Each one:

• typed readonly properties (real PHP type checks, not just PHPDoc)
• static capture(...) factory (constructed from WP — get_post, get_term, pll_get_post_translations)
• static fromArray(array) factory (constructed from a persisted audit row — defensive about missing fields so older audit rows survive)
• toArray() for the JSON wire format (audit log)

Classes

Class	Purpose
PostFieldsSnapshot	Full post for an undoable edit
PartialPostSnapshot	Minimal capture for bulk-update
TermFieldsSnapshot	Term core fields + meta
TermForRecreateSnapshot	Extends TermFieldsSnapshot; adds term_taxonomy_id + object assignments so a delete can be restored under the original id (existing references stay valid)
TranslationLinkBeforeSnapshot + TranslationLinkRow	Polylang group capture

Snapshot stays as a thin facade — abilities mostly want a one-liner. Each helper now returns the typed class (or null when the target's gone) instead of an array.

Reversible accepts both

Reversible::reversible() accepts either an array (for ad-hoc payloads that don't fit a class — restore-redirect, restore-feed, etc.) or a snapshot object and calls toArray() at the wire boundary. That keeps the change focused: snapshots that already had a clear shape get classes; one-off payloads stay as arrays.

RestoreSnapshot

All restore handlers switched to the new fromArray() factories. The defensive parsing in fromArray() means an audit row written by an older release doesn't crash the restore path — missing fields become empty defaults and the caller returns a clean "snapshot was missing X" WP_Error instead of a TypeError.

Tests

Existing RestoreSnapshotTest needed a tiny helper update — restore() now accepts array|object and calls toArray() if needed.

• All 373 tests still pass.
• PHPStan clean at level 6 with no baseline.
• Pint clean.

Why arrays for the inner fields

You'll notice TermFieldsSnapshot::fields is still a typed array (array{name: string, slug: string, description: string, parent: int}) — not a sub-class. That matches the WP-style wp_update_term() argument shape and keeps the array passable to core directly. Composite shapes get classes; raw WP arg arrays stay arrays.

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