v0.35.0

No schema change: x-schema-version stays 1.18. One new error,
E-FREETEXT-DISJOINT, so a lesson that passed 0.34.0 can fail; measured
over the eleven content repositories, none does.

A free_text answer must not also be a distractor (engine#237)

New error E-FREETEXT-DISJOINT: an entry in a free_text exercise's
accept that is also in its distractors, compared after trimming and
case-sensitively. params.shared lists the shared answers, each once.
validateLesson and validateLessonRules (the /rules entry) both report
it. It is the free_text counterpart of E-CLOZE-MS-DISJOINT.

Why: the schema calls distractors the renderer's fallback pool of wrong
options, so a shared entry offers a correct answer as a wrong one. The rule
lived only in the content template's advisory audit, outside CI, the last
content rule there without an engine counterpart. Case-sensitive because two
exercises in adaptive-learner-content teach capitalisation with a distractor
that differs only in case (I am Anna against i am Anna).

A new error can turn valid content red. 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 is found.