feat: add collage layout for 2-3 collection attachments (4/7)

[DeveloperBlue]

Adds the iMessage vertical overlapping collage layout for multi-attachment messages with 2-3 attachments.

Built on top of #3175 (collection building blocks) and continues toward #3147 / #3122.

• New CollectionGroupCollage - overlapping cards with horizontal stagger, vertical overlap, and light tilt
• Route iOS 2-3 attachments to collage layout; 4+ still use the fan stack
• Swipe-to-reply per collage card; reply chevron beside the fixed card frame
• Bugfix: redacted mode leaking contact name / message content in the reply-to input box
• Docs: collections/ routing table + collage ownership notes

2-Item Collage	3-Item Collage	Swipe-to-Reply
		swipe_to_reply.mp4

Changes (commit-by-commit)

1. CollectionGroupCollage - vertical overlapping collage widget (stagger, overlap, tilt, download wrap)
2. Route 2–3 → collage - MediaCollectionLayout + resolveMediaCollectionLayout; MessagePartContent switches collage vs stack
3. Swipe-to-reply on collage cards - collage-local _CollageSwipeCard / SwipeToReplyWrapper
4. Reaction tail split - standard tails on collage cards; inside-tail remains on stack cards
5. iOS stagger polish - author-flush first card for bare 2-ups; subject / 3+ keep first card staggered in
6. Docs - collage layout + routing table in collections/CLAUDE.md
7. Redacted mode reply preview - hide contact name + message content in ReplyHolder when redacted mode is on

Per-file changes

collections/collection_group_collage.dart

New collage layout: sized/overlapped cards, iOS tilt, stagger rules, download wrap, per-card swipe-to-reply.

collections/collection_attachment_card.dart

Reaction tail type: inside for stack, standard for collage (and future grid).

message_part_content.dart

iOS media collections resolve collage vs stack; pass canSwipeToReply into collage only.

helpers/types/constants.dart / helpers/ui/message_widget_helpers.dart

MediaCollectionLayout + resolveMediaCollectionLayout (iOS collage 2–3 / stack 4+; non-iOS grid reserved).

collections/CLAUDE.md

Routing table and collage ownership notes.

text_field/reply_holder.dart

Pass redaction flags into getNotificationText; resolve sender display name via HandleState / fake name when contact info is hidden.

Partially addresses #3122 and #2605 and lays groundwork for #2812