v0.4.0: Reorganize tests for subtree layout + API compatibility

[tazarov]

Goal

Relocate and reorganize tests to align with the new package layout while retaining strong API compatibility coverage.

Scope

• Move implementation-focused tests near moved packages.
• Keep root-level compatibility tests for public API contracts.
• Preserve current integration behavior validation.

Acceptance Criteria

• Tests reflect new package ownership cleanly.
• Public API compatibility tests remain at root.
• make test passes with updated structure.

[tazarov]

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

[tazarov]

Completed: Implementation tests live in internal/runtime/ alongside their packages. Root-level compat_test.go gates the public API surface at compile time. make test passes.