chore(contract): sync flipcash2 protos to 9d44350 (0.18.0)

[bmc08gt]

Syncs flipcash2 protos from 3bb442d3 to 9d44350b (flipcash2-protobuf-api #135, #136, #137, #138) and adds the 0.18.0 changelog entry. Only chat.v1 moved; blob.v1 changed only in comments.

Renumbered: EditChatResponse.Result gets COVER_PICTURE_BLOB_NOT_ACCEPTED inserted at 5, which moves DESCRIPTION_MODERATED from 5 to 6. 0.17.0 shipped DESCRIPTION_MODERATED = 5, so 0.17.0 clients misread a cover-picture rejection and don't recognize 6. This is adopted as merged upstream. StartChatResponse.Result appends its new case at 7.

Renamed (same field numbers, source-breaking): the group picture becomes profile_picture on Metadata, both group-chat parameter messages and EditChatRequest (Picture → ProfilePicture), along with MetadataUpdate.PictureChanged → ProfilePictureChanged and PICTURE_BLOB_NOT_ACCEPTED → PROFILE_PICTURE_BLOB_NOT_ACCEPTED.

Added:

• Cover pictures: Metadata.cover_picture (= 17, may be unset on feed RPCs), cover_picture on both group-chat parameter messages and EditChatRequest, MetadataUpdate.CoverPictureChanged (= 7), COVER_PICTURE_BLOB_NOT_ACCEPTED.
• RosterUpdate.MembershipChanged (= 3): apply roster_summary by version, keep the cached member list.
• Chat.SampleChatters and SampledChatter.
• Chat.SetFeaturedGroups and Chat.GetFeaturedGroups.

Changed behavior: GetRoster is members-only now. A non-member a group's listener rules admit gets DENIED.

7765668 also regenerates ContractInfo.swift for the new changelog heading (0.18.0-dev).

App adoption PRs, drafts carrying the scaffolding only, with the pin bump to follow the release:

• Android: feat(chat): adopt flipcash2 0.18.0 (cover pictures, featured groups, chatter sampling) code-android-app#1686
• iOS: feat(chat): adopt flipcash2 0.18.0 (cover pictures, featured groups, chatter sampling) code-ios-app#983

Both build against this checkout through the local overrides. After merge, run publish.yml at 0.18.0, then bump the pin on each app PR.