feat(photos): save photos to the timeline

Ports upstream C# e0e198e2 (SavePhotosToTimelineAsync, itself the JS
saveToTimeline). Photos already on our photos volume are moved into the
timeline root via PUT photos/volumes/{vid}/links/transfer-multiple,
batched so no request carries more than ten links and a photo is never
split from its related photos; photos on another volume — from an album
shared with us — are copied via POST volumes/{vid}/links/{lid}/copy, one
request each. That closes the cross-volume copy hole (TS copyPhoto).

The re-encryption is what add_photos_to_album already did, so it moves
into a shared photo_target_payload: name re-encrypted to the target key
and signed by the photos membership address, name/content hashes under
the target hash key, node passphrase rewrapped so the original
NodePassphraseSignature survives.

A photo the server reports an incomplete group for is re-queued once
with the related photos it named; only its second answer becomes an
outcome, so outcomes arrive in completion order, not input order.

Also adds ProtonPhotosClient::enumerate_events (c8d03244's only
non-interop hunk) and bumps the upstream pin to c8d03244. Upstream
29e58378 (upload telemetry) and 0537d906 (NodeProvider refactor) are
deliberately not ported — see UPSTREAM_SYNC.md.

README.md

@@ -302,7 +302,8 @@ neither upstream SDK, original to `proton-sdk-rs` · ❌ not implemented.
 | | Shared Photos | ✅ | Photos/albums shared *with* us (`enumerate_shared_with_me_node_uids`, `enumerate_shared_with_me_album_uids`) and by us (`enumerate_shared_node_uids`). |
 | | Duplicate Detection | ✅ | `find_duplicates` matches candidates on name/content HMAC before re-uploading. |
 | | Favorite Shared Photos | ❌ | Needs the photo re-encrypted for our timeline root; not ported (upstream `03b1cb7f`). |
-| | Album Writes | ❌ | Creating albums and adding/removing photos — no C# public API upstream either. |
+| | Save to Timeline | ✅ | `save_photos_to_timeline` moves photos already on our photos volume into the timeline root and copies cross-volume ones (from albums shared with us). |
+| | Album Writes | ⚠️ | `create_album` and `add_photos_to_album` are ported from the TypeScript SDK (C# has no public album write API); album rename, cover photo and delete are not. |
 | | Photos Volume Create| ❌ | Volume creation is not yet ported. |
 | **Caching** | Pluggable Cache | ✅ | Pluggable entity cache (`with_entity_cache`, or `with_entity_repository` to chain one onto a `with_key_salts` client); keys/secrets remain strictly in memory. Retained deliberately — upstream C# removed its entity cache. |
 | **Validation** | Node Names | ✅ | Empty or >255-char names are rejected client-side before create/rename (counted in `char`s; C# counts UTF-16 units). |