fix(tutor): let serial-output be learned without an int variable

[ttbombadil]

Problem

variables-and-serial is activated by any Serial.print* call. Its Concept serial-output required variable-values, and all questions of variable-values apply only to sketches that use int.

On sketches that print via Serial but have no int, three things followed:

• serial-output could never be selected;
• the Topic was unresolved from the first turn;
• UnoSim blocked the Topic-driven Tutor with content-exhausted for the whole session (LearningQuestions SSOT 2.3).

This affected 16 of 35 Examples:
it01-02-konstante, it02-03-boolesche-operatoren, it02-04-bitoperatoren, it03-01-serielle-ausgabe, it03-02-serielles-zeichen-einlesen, it03-05-millis (after long-values), it05-03-switch, it06-01-while, it06-02-do-while, it06-03-for, it06-04-verschachtelte-schleifen, it07-01-funktion-ohne-rueckgabe, it08-01-struct, it08-02-array-aus-structs, it08-03-struct-als-parameter, it08-04-funktion-auslagern.

The existing quality gate did not see it, because it only checks Topics that a quality case expects.

Change

serial-output.prerequisites: [variable-values] → [], plus the Topic's SHA-256 in tutor/manifest.yaml. Nothing else changes.

Why this is the right fix and not a test workaround:

• The Concept's objective, indicator, misconception and scaffold depend only on Serial output.
• Both of its questions (concept, transfer) require only serial-call: print; none mentions an int.
• Activation, Concepts, questions and mastery are unchanged.
• preferredOrder: [variable-values, serial-output] still teaches variable-values first wherever the sketch has an int.

Evidence

All checks are deterministic: the real UnoSim TutorService, CurriculumTutorAdapter, matcher and planner, with a fake provider standing in for the model. No provider calls.

	before (2ac716f)	after
Topic activations unresolved from the first turn	16 / 36	0 / 36
it06-03-for (strong learner)	no planned question, blocked from turn 0	serial-output-concept → serial-output-transfer → mastery
it03-05-millis	blocked after long-values mastery	long-values and variables-and-serial both mastered
it01-01-variable-speichern	unchanged	unchanged
validator at the pinned UnoSim commit (aaa52fad, this repo's CI)	pass	pass
new catalog-wide UnoSim authoring gate (MoDevIO/UnoSim, open PR)	16 × unmasterable-example-activation	pass

Known and out of scope: after mastery, post-mastery content is thin. Serial-only Examples have no DEEPEN question left and continue with the blocked free fallback. That is legitimate per the runtime contract and a separate authoring task (more DEEPEN questions, Topic v2 extensions).

Follow-up

Once the UnoSim gate PR is merged, bump .unosim-compatible-commit so that this repository's CI enforces the catalog-wide check.

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