refactor: remove resolution-epic dead code and document the new resolution surfaces

[TKaltofen]

Summary

Post-epic cleanup for the FeatureGroup resolution work, in two ordered steps: remove dead code left after the epic, then document the new resolution surfaces against what remains.

Dead code removed

• FeatureChainParser._match_pattern_based_feature: only self-referenced; the docstring claim that match_configuration_feature_chain_parser used it was stale.
• format_feature_group_classes (plural) and its dedicated test file. The singular format_feature_group_class stays (used by the engine and covered by the error-message tests).
• The log_n_subclasses parameter and its unreachable, buggy debug branch in get_all_subclasses.
• Options.add, a zero-caller legacy migration shim.

Behavior-preserving tidy-up

get_column_base_feature now splits on the exported COLUMN_SEPARATOR constant instead of a bare "~", giving the constant an internal consumer. COLUMN_SEPARATOR == "~", so behavior is identical.

Diagnostic seam blessed (not retired)

_strict_validation_rejection_reason and its helper name_path_presence_rejection_reason are the assertion seam across roughly a dozen test modules. Rather than retire them, their docstrings now mark them as a supported diagnostic seam (docstring-only, no behavior change), mirroring the os-014 "bless the test seam" precedent.

Docs

• Troubleshooting page: resolution failures are reframed around the typed FeatureResolutionError (a ValueError subclass) with a catching example.
• mloda-api.md: diagnose documented next to explain, resolution_report() next to resolved_plan(); the stale resolve_feature block replaced with a pointer to the accurate discover-plugins.md; FeatureResolutionError, ResolutionDiagnosis, and ResolutionRecord documented.
• One-line docstrings added to mlodaAPI.get_result() / get_artifacts().
• memory-bank/activeContext.md and progress.md refreshed for the finished resolution epic.

Breaking changes (public API)

Two symbols on mloda's public surface are removed. Both were explicitly sanctioned for removal by the ticket and verified zero-caller across mloda core and mloda-registry:

• Options.add(key, value) (reachable via from mloda.user import Options) is removed. Use Options.add_to_group(key, value) instead, or add_to_context(key, value) for metadata-only options.
• get_all_subclasses(cls, log_n_subclasses=...) (exported via mloda.provider) loses its log_n_subclasses parameter. No internal call site passed it, and its only effect was an unreachable debug branch that indexed the subclass set rather than logging a count. Call get_all_subclasses(cls).

Validation

tox green (pytest -n 8, ruff format + check, pip-licenses, mypy --strict, bandit). The two public-API removals above were reviewed for backward-compatibility impact and are intentional per the ticket.

[TKaltofen]

Follow-up commit: restored the 3 direct unit tests for the singular format_feature_group_class, which stays live in production (engine, execution plan, feature-group step). Deleting the plural's dedicated test file had also removed the only direct coverage of the singular formatter (the error-message tests exercise it only incidentally). New file: tests/test_core/test_abstract_plugins/test_feature_group/test_format_feature_group_class.py. tox green (7175 passed, 170 skipped).