feat(validate): a free_text answer must not also be a distractor (#237)

[astrapi69]

Closes #237.

Summary

New error E-FREETEXT-DISJOINT in src/rules.ts, the free_text counterpart of E-CLOZE-MS-DISJOINT.

• What it catches: an accept entry that is also in distractors.
• Comparison: after trimming, case-sensitive.
• Params: shared lists the shared answers, each once.
• Reported by: validateLesson and validateLessonRules (the /rules entry).

Case-sensitive because two exercises in the hub teach capitalisation with a distractor that differs only in case (I am Anna against i am Anna). The issue text wrote i am anna; the real distractor is i am Anna.

Docs

• lesson-format.md:
  • the free_text section explains distractors and the rule;
  • the rule catalog and the issue-parameters table have the new id.
• validation.md: the free_text row.
• architecture.md: an open item. Engine side done; the template's audit drops its copy after the release and re-pin.
• CHANGELOG: [Unreleased] entry.

Test plan

• src/free-text-disjoint.test.ts: 9 tests, red before the change:
  • reproduction;
  • happy path, message and anchor, the /rules entry;
  • edge cases: no distractors, empty distractors, an entry repeated in accept;
  • boundaries: whitespace counts as the same entry, case does not.
• src/issue-params.test.ts: a params case for the new id. The params-table gate was red until the docs listed it.
• make release-check: 1759 tests passed.
• make prose-check and npm run docs:api:check: green.
• Measured with this build over the 632 lessons on origin/main of the eleven content repositories: 0 hits. An overlap seeded into a real lesson (adaptive-learner-content, ex-free-i-capital) is found at /steps/9/exercise.

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