W27 — v0.4 consumer diagnostics

[ammarheidari]

Objective

Add evidence-based consumer diagnostics without fabricating metrics or history.

Deliverables

• inactive/stalled diagnostic model,
• optional metrics observation port,
• optional history observation port,
• explicit unavailable/partial states,
• bounded observation comparisons,
• deterministic tests.

Default deployment has no mandatory metrics/history backend.

Depends on W26 consumer observations.

Central tracker: #100
Scope authority: #99 / RFC-0004.