Lists/Communication: SendToAudience loads only PK + recipient + name — templates cannot see row fields

## What's wrong

`SendToAudience` resolves an audience and fans out per-recipient messages, but its bulk load projects only `[primary key, RecipientField, FullNameField]`. The per-recipient `ContextData` is set to that loaded row — so message templates cannot reference any other column of the audience rows (e.g. a per-row token, company, status, or URL), which defeats per-recipient personalization for most real audiences.

Related sharp edge worth fixing or documenting in the same pass: in `Engine.SendMessages`, a recipient's `ContextData` wholesale REPLACES the base message's `ContextData` (even when `undefined`) rather than merging — shared base context must be duplicated into every recipient by the caller.

Found while evaluating SendToAudience for "email N invite rows with per-row unique links" — the projection made it a non-starter despite the audience/fan-out machinery being exactly right.

## Proposed shape

Either load the full row for `ContextData`, or (better for wide tables) accept an optional field-projection list on the audience source / send call. Document or fix the replace-vs-merge semantics for recipient ContextData.

## Acceptance criteria

- [ ] A SendToAudience template can reference arbitrary fields of the audience entity's rows (full row or explicit projection)
- [ ] ContextData replace-vs-merge behavior is explicit (documented or merged with a defined precedence)
- [ ] Existing SendToAudience consumers are unaffected by default

## Blocked by

None - can start immediately