[#457] Add exhaustive typed pool error taxonomy

[arisu6804]

Summary

This PR adds the exhaustive typed pool-error taxonomy requested in #457.

• preserves all existing append-only RouterError codes;
• adds typed class/name metadata and retry/configuration remediation guidance;
• adds an on-chain get_error_catalog discovery surface;
• adds exact code round-trip and unknown-code handling;
• adds exhaustive operation, class, retryability, and boundary metadata tests;
• documents every code, negative-path matrix, client handling rules, and review checklist.

The change set contains 599 added lines of implementation, tests, and documentation.

Validation

• git diff --cached --check
• cargo test --lib error_taxonomy::tests — 14 passed

Closes #457