#25774 acpmux docs: the prompt-uploads contract (files of any size, by reference)

The contract for sending files of any size with a prompt, by reference, for the native upload worker and the web composer to build against. Docs only; split out of #25730 so that PR stays near 800 changed lines.

Leo: "why the FUCK SHOULD THERE BE A FUCKING SIZE CAP ANYWAYS JUST FUCKING / I ALREADY SAID WE NEED ARBITARRILY LARGE FILES OF ANY TYPE"

What it covers (cmux-tui/crates/acpmux/docs/prompt-uploads.md):

• Section 0: what exists at which head. The resumable file_put_* calls with no cap, upload-ref blocks and the promptUploads flag come with #25730; binary data frames, file_put_status, finish sha256 and the upload.* reasons are a follow-up behind the uploadFrames flag.
• Section 1 / 1b: the binary frame format, and how bytes a page holds reach native over DevTools without touching the page binding.
• Sections 2-7: what the daemon checks of the caller, the calls, errors (a full disk is the only size failure and names host and path), the two exact ref shapes, what the agent receives, idempotency.

Risk: none at runtime (Markdown only).

Agent session: agent session

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