feat: split collection stack into recomposable collection building blocks (3/7)

[DeveloperBlue]

Breaks the monolithic MessageImageGallery fan into reusable collection widgets so later layouts (collage, grid, etc.) can share the same cards, title, and download chrome. Also renames gallery-specific flags to collection naming, and moves clip/shadow/gesture ownership onto those collection components.

Built on top of #3166 (gallery fan refinements / stack-base) and continues toward #3147 / #3122.

• Replace MessageImageGallery with collections/ - CollectionGroupStack, CollectionAttachmentCard, CollectionTitle, CollectionDownloadButton
• fill is the sole collection flag on AttachmentHolder - parents own clip/shadow/corners
• Rename gallery → collection (isMediaCollection, isInCollection, docs) so the shared collection API isn’t stack-specific
• Move long-press / popup onto per-card CollectionAttachmentCard instead of the whole stack wrapper
• Per-attachment tapbacks with inside-tail reaction bubbles on for the stack (ios parity)
• Reusable circular download button for incoming iOS collections
• bugfix: Popup scope sizes, bounds, and aligns collection-card previews (wider maxWidth, horizontal clamp, no fill expand in popup)
• bugfix: subject + multi-attachment messages render correctly after collection collapse

Before	After
Incoming download button + inside-tail tapback
bugfix: Subject + multi-attachment grouping

Changes (commit-by-commit)

1. Rename gallery flags → collection - isMediaGallery → isMediaCollection, isInGallery → isInCollection, related docs/comments
2. Subject + multi-attachment fix - collapse/render path keeps subject + media collections correct
3. Split MessageImageGallery - extract CollectionGroupStack, CollectionAttachmentCard, CollectionTitle; delete monolith gallery widget
4. Per-card long-press - popup holder moves onto collection cards; stack wrapper stops owning press
5. Popup scope for collection cards - measure width/height, clamp overlay/preview, grow maxWidth to ≥ 50% screen; AttachmentHolder skips fill expand in PopupScope
6. Inside-tail tapbacks - reaction clipper/holder support collection-card alignment; reactions rendered per card
7. Rename partIndexForAttachment → partIdForAttachment - clarify id vs list-index naming
8. Collection download button - reusable control for incoming stacks
9. Stack spacing polish
10. Docs - attachment/ + collections/ CLAUDE ownership notes
11. Lift shadow/clip onto card - CollectionAttachmentCard owns chrome
12. fill sole presentation switch - drop transparentBackground / showCardShadow from AttachmentHolder
13. Document fill - holder + collections docs

Per-file changes

collections/collection_group_stack.dart

Fan stack (ex-MessageImageGallery): 3:4 cards, swipe/drag, title row, download wrap.

collections/collection_attachment_card.dart

Card chrome (shadow/clip), popup, AttachmentHolder(fill: true), per-card reactions.

collections/collection_title.dart

“X Photos / Videos / Items” label above the stack.

collections/collection_download_button.dart

Circular download control for incoming iOS collections.

attachment_holder.dart

fill is the only collection presentation flag; popup-aware expand; reserved-box/compact placeholders kept for standalone downloads.

message_popup.dart / message_popup_holder.dart

Collection-card popup sizing/alignment; press ownership on cards.

message_part_content.dart / message_holder.dart

Route iOS collections through CollectionGroupStack; reaction/index wiring updated for collection naming.

reaction*.dart

Inside-tail bubble clipping/placement for collection cards.

message_part.dart

Collection naming + partIdForAttachment.

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