refactor(cli): extract table rendering into cli/rendering.py

[ArtVsMark]

Summary

Phase 1 (safe rendering extraction) of #121, Stage 1 of the cli.py decomposition epic (#117). Branched from fresh main (#118–#120 already merged).

Moves the pure csv/markdown rendering helpers — _rows_to_csv, _rows_to_markdown, _print_tabular — out of cli/__init__.py into a new leaf module cli/rendering.py, re-exported from the facade for backward compatibility. Same shape as #119's options.py and #120's commands.py extractions.

_print_tabular is already routed through CliContext.print_tabular (added in #120), built fresh per call by _build_cli_context() — mode-handler callers in cli/commands.py are unaffected by this move.

Test fallout (found by running the regression net, not assumed)

_rows_to_csv/_rows_to_markdown are only called internally by _print_tabular (now co-located with them in rendering.py), not referenced anywhere else in cli/__init__.py's body post-move — the same "hidden facade dependency" shape as #119's CONFIG issue. Two tests in test_cli.py (TestFacadeNamespaceContract) patched cli._rows_to_csv/cli._rows_to_markdown expecting the effect to propagate through cli._print_tabular(...); since that internal call now resolves within rendering.py's own namespace, patching the facade-level re-export no longer reaches it. Updated both to patch cli.rendering._rows_to_csv/cli.rendering._rows_to_markdown instead — matching the cli.options.CONFIG pattern from #119.

Every other monkeypatch-based test in the suite needed no changes.

Scope note

This is Phase 1 only. Phase 2 (_LANG/_MESSAGES/_LOCALE_MESSAGES/_t/_interactive_menu/_ask_*/_pick_path_via_dialog) is explicitly out of scope per #121's own non-goals — it needs a separate CliContext-style design pass, same as #120 required.

Test plan

• pytest tests/ -q — 842 passed, 3 skipped (2 test lines changed in test_cli.py, discovered by actually running the suite before committing)
• ruff check src tests docs / ruff format --check src tests — clean
• mypy src/stepik_grader --ignore-missing-imports — clean
• Manual smoke: stepik-grader --mode 1 --output csv/markdown against a real solution+tests dir
• Updated docs/architecture.md (module table + DAG) and docs/project-structure.md

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