refactor: default App to generic profile

[codeforester]

Summary

• make base_cli.App() select CliProfile.generic() when no profile is supplied
• keep CliProfile.legacy_base() as an explicit, temporary migration surface
• update the consumer-profile docs and changelog
• make Base-specific compatibility tests opt into the legacy profile explicitly

This is the first implementation slice of #19. Base already supplies an explicit profile, so its consumer behavior is unchanged.

Validation

• git diff --check
• uv run --extra dev pytest
• 159 tests passed, plus 4 subtests

Fixes #20
Part of #19