Docs: EQL v4 and Stack Encrypt compatibility policy #1314

Write down the guarantees long-retention users depend on (e.g. 7-year retention):

• Within EQL v4, a domain type's wire format is stable across minor and patch releases. A breaking change ships as a new domain type.
• Stack Encrypt and EQL are versioned independently. Each Stack Encrypt release declares the minimum EQL version it supports (LTS).
• How breaking changes are communicated.

Milestone coordination

Included in milestone 4. #1490 owns the overall stable Rust release gates and supported profile/backend matrix; this issue remains the owner of the EQL/persisted-data compatibility policy. #1478 supplies cross-version executable evidence.