Context

Part of #685. Derived from the 2026-06-13 multi-agent ecosystem analysis. The existing conformance linter (scripts/conformance_lint.py, #693) checks structure/metadata only and gates none of the cross-system contract violations the analysis found.

Problem

The centralized error-code registry (disjoint negative ranges per system) is unenforced convention:

• container/monitoring emit POSITIVE codes into common's error_info.code (silently classified "Success")
• database mints a local enum colliding with common's reserved -5/-6
• logger/network fallback paths define structurally incompatible Result/error_info types

None is caught by CI. Additionally error.cppm re-declares the entire registry inline (not #include of error_codes.h), so a remap that edits one but not the other silently desyncs module-built vs header-built consumers.

Scope / Acceptance Criteria

• Compile-time static_assert in each consumer repo: every public error code is < 0 and within that repo's reserved window
• Runtime test: to_common_error() round-trips and never yields a Success-classified code from a real error
• Structural compatibility check for Result/error_info fallback types (logger/network no-common paths)
• error.cppm vs error_codes.h constexpr-declaration diff gate (fail on mismatch), OR refactor error.cppm to #include the header (single SSOT)
• DoD: a deliberately-bad fixture (positive/out-of-range code) FAILS the build in the introducing repo
• Gate wired as a required status check (staged rollout per the propagation epic)