acpmux: remove the 15 MiB message cap
Leo asked: “Why is there a 15 MB cap and why are we not following protocol/transport best practices?”
This removes the 15 MiB acpmux message ceiling by aligning the safe inline message budget with the transport frames already used by the app and daemon: 31 MiB for JSON-RPC messages, with a 28 MiB prompt-inline budget for envelope and relay metadata. Existing staged uploads and zlib frame compression remain in use for larger or attachment-heavy payloads, so arbitrary-size files continue to travel by reference instead of being forced into one frame.
The transport comments, wire-size fixture, and prompt budget tests now exercise the larger limit with a 20-screenshot batch and copied attachments. This keeps protocol frames bounded while eliminating the user-visible 15 MiB hard stop.
Validation: focused Bun prompt-budget tests and acpmux Rust transport tests.
Agent session: agent session
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Summary by cubic
Removes the 15 MiB acpmux message ceiling to match the transport frames the app and daemon already accept. acpmux now takes one JSON-RPC message up to 31 MiB, with a 28 MiB prompt-inline budget for the envelope and relay metadata. Staged uploads and zlib frame compression still carry larger or attachment-heavy payloads, so files keep traveling by reference.
Tests now exercise the larger limit: a 20-screenshot batch, 15 image copies, reply-fit fixtures sized from the reply budget, list/page fixtures scaled up to stay past the bound, and peer-link fixtures using the large frame config with a 32 MiB oversized record.