Extension sources are represented by one tagged value

[jsmestad]

Parent Epic: #2813

Type: Chore

Related work: #211

What

Extension.Entry stores a source_type discriminator beside nullable path, git, hex, and module fields, allowing contradictory and incomplete source combinations.

Why

Tagged source data makes invalid combinations unrepresentable and gives install, compile, update, lazy-load, and display paths exhaustive matches.

Acceptance Criteria

1. Extension.Entry carries one tagged source value with the required payload for path, git, Hex, or module sources.
2. Constructors reject incomplete source data structurally rather than through downstream guards.
3. Loaded runtime module identity remains separate when it is not part of source identity.
4. Install, compile, update, lazy-load, display, persistence, and reload behavior remains compatible.
5. Table-driven tests cover every source variant and reject invalid construction.

Developer Notes

Approach: Replace source_type plus nullable companion fields with tagged tuples or focused source structs. Put source-specific accessors and transitions on the owning value.

Files/areas: lib/minga/extension/entry.ex, lib/minga/extension/supervisor.ex, lib/minga/extension/lazy.ex, extension source tests

Testing: Run extension path, git, Hex, module, lazy-load, reload, update, and persistence suites.