[codex] docs: record quote observation decision

[Egge21M]

Summary

• Adds Quote Observation to the domain glossary.
• Clarifies that newer UNPAID melt quote observations can supersede older PENDING observations.
• Adds ADR-0004 recording that quote observations are persisted before quote-backed operations advance.

Why

The melt quote observation slice has landed via #273, and the active melt watcher/settlement issues already refer to this vocabulary and ADR. This keeps the repository context aligned with the implementation and with PRD #267.

Impact

Documentation-only change. No runtime behavior, API surface, schema, or package release metadata changes.

Validation

• git diff --cached --check before commit

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Latest commit: db11003

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