docs: I-10 reclassified incidental -> design-joint with Thalamus frequency data (universal precondition, fold-coupled HARD, joint projection fix)

Author: ualtinok

.alfonso/audit-cc-parity-gaps.md

@@ -90,7 +90,7 @@ These can be implemented on the Claude Code leg without changing the harness's f
 - **OpenCode/Pi:** only system messages inside the current compartment coverage interval are copied into m0; an older uncovered leading system message remains a separate leading message.
 - **Claude Code:** every covered system message before the coverage end is copied into m0 regardless of coverage start, any coverage advance over a system message forces a HARD, and covered leading system messages are never separately re-emitted.
 - **Source:** force-HARD gate at `crates/mc-module/src/transform.rs:2987-3005`; m0 collection at `crates/mc-module/src/transform.rs:5153-5178`; separate-message gate at `crates/mc-module/src/transform.rs:9321-9339`.
-- **Classification:** incidental — the gateway can project CK system items into Anthropic's top-level `system` field while preserving the OpenCode/Pi interval and timing.
+- **Classification:** DESIGN-JOINT (reclassified 2026-08-14 with Thalamus wire data): the precondition is universal on this leg — 750/751 real requests carry interior system messages (median 10, max 36; 17.8% of array bytes) — so every historian publish advance crosses a system ordinal and every fold becomes a system_absorb HARD (fold-frequency-coupled, not pass-frequency-coupled; verified at transform.rs:3058-3071, gated on compartment_seq_changed_since_meta). The HARD is load-bearing today: on the byte-splice-suppressing profile it is what moves covered system bytes into m0 before separate re-emission is suppressed (9321-9339) — dropping it loses system content. Fix is the joint move: gateway projects covered CK system items into Anthropic's top-level system field (preserving OC/Pi interval semantics), then the absorb-HARD gates off for profiles with system projection. Must flip together, epoch-coordinated like the U0 absorb introduction. Queued behind parity R3.
 
 ## I-11 — Claude Code's cache-lifetime policy differs from OpenCode/Pi