Remove comments referencing invisible external context

[Moferanoluwa]

Change Description

Fixes #1500. Three spots in the toolchain referenced context that means nothing to a reader of the public repo:

1. toolchain/mfc/viz/tui.py — 10 occurrences of (Feature N) / Feature N — tags referencing a private feature checklist not present in this repo. Tags stripped, descriptive text kept (capitalized where the tag had been the leading word of a docstring).
2. toolchain/mfc/case_validator.py — removed an orphaned # Justification: ... comment sitting after the module docstring, which read like a leftover review note rather than documentation.
3. toolchain/mfc/params/definitions.py — removed a migration-note sentence referencing a definitions/ directory that no longer exists in this repo.

No logic changes; comment/docstring text only.

Testing/Review Recommendations

• python3 -m py_compile on all three touched files passes.
• ruff format --check passes on all three files (no reformatting needed).
• ruff check against .ruff.toml reports 3 pre-existing PLR0917 (too-many-positional-arguments) findings in this repo, all on function signatures I didn't touch (tui.py:451, tui.py:572, definitions.py:521) — confirmed these are present on master as well, unrelated to this change.

AI Usage

• Tool: Claude Code (Claude Sonnet)
• Type: Locating each referenced line, applying the exact edits described in the issue, and verifying no other occurrences were missed.
• Scope: The three files listed above; comment/docstring text only, no behavioral changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.04%. Comparing base (62e4783) to head (ec2bda8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1699   +/-   ##
=======================================
  Coverage   61.04%   61.04%           
=======================================
  Files          83       83           
  Lines       20978    20978           
  Branches     3099     3099           
=======================================
  Hits        12807    12807           
  Misses       6126     6126           
  Partials     2045     2045           

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