docs(uploads): note that marked fields are multipart-only

[adnaan]

Docs half of livetemplate/livetemplate#508. Pairs with livetemplate/client#152.

lvt-upload-with was documented as a contract without naming the transport it holds on. It holds on the multipart path — always for Proxied (so OnUpload always sees marked fields), and for Volume/Direct only when the socket is down. Over the chunked WebSocket transport the fields never arrive.

The new subsection says so, notes the client now warns at that point, and — more useful than the prohibition — points at what to do instead: read context from the controller state you already hold rather than from the upload's form.

No behaviour change here; this is the boundary the client PR makes diagnosable.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Ui2cwpeGkrUfRt8rh2FgGG

[adnaan]

Correction pushed. The first pass claimed "Volume and Direct uploads go over the WebSocket in chunks" and that marked fields "do arrive if the socket is down."

Both are wrong for Direct, which routes to uploadExternal — a presigned PUT plus a metadata-only completion, touching neither the chunked nor the multipart path. The socket-down sentence was the damaging one: it reads as a workaround, and a reader following it would drop their WebSocket and still find empty values.

Replaced with a per-mode table (Proxied always / Volume on the fallback only / Direct never) and an explicit note that Volume's intermittent delivery is its own trap — a handler that sees fields sometimes is worse to debug than one that never does.