feat: resolve group message senders to room-specific nicknames

[eyaeya]

Summary

• Resolve group-message senders against room-specific RoomData nicknames while preserving stable sender IDs and contact-level display names.
• Add a strictly bounded, same-room quoted-message fallback for cases where contact.db is unavailable or unreadable, or the member is absent from current RoomData.
• Apply the identity contract consistently to messages/timeline/history, context, search, and media metadata.

Output contract

For group messages:

• sender_wxid remains the stable identity.
• sender_group_nickname is emitted only when a room-scoped nickname is resolved reliably.
• sender_contact_display preserves a distinct contact-level display name when available.
• sender prefers group nickname, then contact display, then raw sender ID.

Private messages retain their existing behavior. Unresolved optional fields are omitted.

Implementation notes

• Decode only the required subset of the chat_room.ext_buffer RoomData protobuf.
• Treat current RoomData membership as authoritative even when its display name is empty, preventing an old quote from resurrecting a cleared group nickname.
• Use the newest bounded historical quote scan before falling back to identity evidence already present in the requested rows.
• Require the carrying room to match refermsg.fromusr, reject room IDs as member IDs, and filter fallback results to requested members.
• Bound RoomData sizes, member counts, room counts, message sizes, per-shard scans, and request-wide scan volume.
• Perform group-nickname search enrichment after filtering and pagination.
• Degrade to the existing contact/raw-ID display if any optional lookup or parse fails.

Validation

• go test -count=1 ./...
• go test -race -count=1 ./cmd/wechat-cli
• go vet ./...
• git diff --check
• Windows amd64, CGO_ENABLED=0 cross-compilation of ./cmd/wechat-cli
• macOS arm64 with WeChat 4.1.12 in strict read-only mode, using two private local fixtures

All committed tests use synthetic room, member, nickname, and message values.

Closes #15