cp: enforce MIN_RUNTIME_FRAME_BYTES with its first consumer and document the breaking config floor #1590

Follow-up from the round-3 review of #1588 (finding F2): #1588 (comment)

Problem

#1588 adds pub const MIN_RUNTIME_FRAME_BYTES: usize = 1024 * 1024; in crates/openab-cp/src/proto.rs. Nothing uses it yet: there is no consumer, no enforcement and no test. Its doc comment says CP config validation will start rejecting smaller max_frame_bytes values in the runtime-client PR (PR B of the #1528 split).

Once that lands, an existing CP config with max_frame_bytes below 1 MiB, which validates today, will fail validation after upgrade. That is a breaking config change.

Proposed fix (in the runtime-client PR)

• Add the validate() floor for max_frame_bytes >= MIN_RUNTIME_FRAME_BYTES together with its first consumer (the runtime client).
• Test it with an exact-message assertion: ASCII message, and no claim of frame-size negotiation (see feat(cp): land Agent Control Plane v1 on main #1528 round-3 F7).
• Call out the floor as a breaking config change in the release notes / PR description.
• If the runtime-client PR does not land, remove the unused constant.

Acceptance criteria

• MIN_RUNTIME_FRAME_BYTES has at least one consumer and is enforced by CpConfig::validate(), with a test.
• The breaking config change is documented for operators.