docs: cross-link ARCHITECTURE.md from each subpackage docstring

[JohnnyWilson16]

Problem

ARCHITECTURE.md (added in #158) maps the src/freshdata codebase, but nothing inside the source points readers to it. Someone browsing a subpackage's __init__.py has no breadcrumb back to the map.

Note: depends on #158 merging first (that PR adds ARCHITECTURE.md).

Expected outcome

Each subpackage __init__.py docstring ends with a one-line pointer, e.g.:

See ARCHITECTURE.md for how this package fits into the overall cleaning flow.

Relevant files

The __init__.py of each subpackage listed in ARCHITECTURE.md: engine/, steps/, imputation/, domains/, semantic/, context/, enterprise/, compliance/, execution/, streaming/, parsers/, render/, models/, learning/, integrations/, adapters/, experimental/.

Acceptance criteria

• Docstrings only — no behavior change.
• ruff check . and mypy src/freshdata pass.
• The fast test lane (pytest -m "not online and not large") stays green.

Difficulty / skills

good first issue — Python, orientation reading. A safe, mechanical first PR.