v0.4.0: Add compatibility gate checklist for root cleanup refactor

[tazarov]

Goal

Define and execute release compatibility checks for the non-breaking layout refactor before merging.

Scope

• Validate public API parity (method signatures and behavior).
• Validate runtime artifact behavior unaffected by package move.
• Run smoke verification for downloader/local embedded paths.

Acceptance Criteria

• Explicit checklist completed before merge.
• No import-path break for current users.
• Release notes include refactor summary and compatibility statement.

[tazarov]

Tracked by umbrella #45 (v0.4.0 root-cleanup plan).

[tazarov]

Completed: compat_test.go serves as the compile-time compatibility gate — every exported type, function, constant, and variable is checked. Import path github.com/amikos-tech/chroma-go-local remains stable.