Supersedes ADR 0030's 'types, not behaviour' line for mesh-sdk. The
boundary is change-frequency, not kind: the SDK holds the stable spine
(tool-serving harness, messaging/event framework, contracts, core
primitives) and refuses per-module clients, per-module tool code, and
anything volatile — because those are what turned hal/sdk into constant
maintenance and made every edit rebuild every module.
States the rule (frequent AND cascading is the disease), why the root
cause was intra-module feature-sharing leaking into inter-module
coupling, and where per-module shared code lives instead (in the
module — a shared file, or a module-local sdk for the few large ones).
Updates repos.md's canonical mesh-sdk description to match; leaves 0030
untouched (immutable).
Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF
