Refactor Astro AtProto Loader to ex;lrt defineAtProtoCollection and defineAtProtoLiveCollection

Author: essential-randomness

.changeset/atproto-loader-extravaganza.md

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+---
+"@fujocoded/astro-atproto-loader": minor
+---
+
+Rework the loaders for better ergonomics and typing. Featuring breaking changes!
+
+- `atProtoLiveLoader` → `defineAtProtoLiveCollection`
+- `atProtoStaticLoader` → `defineAtProtoCollection`
+
+New capabilities:
+
+- Multi-source pipeline: load from several `repo` + `collection` pairs in one collection, with `value` discriminated on `collection` inside `filter`/`transform`.
+- Per-source `parseRecord` for `$parse`-style lexicon validation; failures drop the single record without failing the source.
+- `groupBy` + grouped `transform` for merging records across sources under a shared key (e.g. post + reposts).
+- Shared `fetchRecord` hydrator passed to every callback, with per-cycle request deduping.
+- `onSourceError` policy (`'throw' | 'skip' | fn`)
+- `limit: number | 'all'` and `maxPages` for explicit pagination control.