docs: reorganize backend design guides

[zzylol]

The four backend design guides mixed dense protocol prose, repeated implementation phases, and acceptance requirements without a complete example near the start. This rewrite gives each document the same reader-first structure: an overview, one worked example, the reusable contract, and concise acceptance or limitation sections.

Changes:

• Reorganize the ASAPQuery compatibility profile around one startup-to-query example, explicit profile boundaries, runtime contracts, and one executable evidence matrix; remove duplicated component/phase narratives.
• Add a measured-evidence example and clarify artifact, ownership, comparison, and acceptance rules for offline o11y evaluation.
• Add a shape-observation and ERP-match example, then separate matching, cost composition, fallback, payload migration, and population isolation.
• Expand the previously dense completeness prose into explicit admission, publication, query-fence, drain, recovery, and limitation contracts with a partial-publication example.

The changes preserve the documented behavior and scope. They do not change runtime code or claim new capabilities.

Validation:

• git diff --check
• local Markdown link and anchor validation, including inbound links from docs/