feat(manifest): optional set-entry visibility flag (Closes #83)

[astrapi69]

Summary

Adds an optional visibility: "visible" | "hidden" flag (default "visible") to the content-manifest set entry, so content repos can mark technical reference / conformance fixtures as not-for-learners without an app-side blocklist.

The motivating case is the graded-quiz demo in adaptive-learner-content-test, the deliberate E-EXT-UNSUPPORTED negative case in scripts/conformance-real.mjs: it must stay on disk for conformance but should not surface to learners. The strict set-entry schema (additionalProperties: false) previously rejected any such metadata, forcing the interim hardcoded blocklist (frontend/src/lib/content/repos/hidden-sets.ts) in the consumer app - the wrong layer for repo-owned metadata.

Critical semantic preserved: visibility is a consumer-display hint only. The engine and conformance-real.mjs still fully validate hidden sets and never exclude them from validation or conformance; only consumer apps filter on it. Absent means visible, so every existing manifest keeps validating unchanged.

What changed

• schema/content-manifest.schema.json: optional visibility enum on ContentSet, default "visible", additionalProperties: false retained.
• Canonical projection: asContentSetEntry exposes it on ContentSetEntry.visibility (new SetVisibility type), normalizing any out-of-enum value defensively back to "visible" (fail-safe: an unknown value stays visible rather than silently hiding content).
• Exports added to both barrels; brief note in docs/concepts.md; CHANGELOG entry.

Design decisions (left open by the issue)

• visibility enum over a boolean dev_only - more expressive and forward-compatible.
• x-schema-version stays 1.8 - additive optional field, backward compatible, matching the field-level additive precedent in the lesson schema. The frozen schema baseline is refreshed in the same commit.
• Version bump 0.13.3 → 0.14.0 (minor) - new public API surface.

Out of scope (downstream chain)

The downstream steps live in other repos and are not part of this engine change: setting the flag on the graded-quiz set in adaptive-learner-content-test, and reading it / deleting hidden-sets.ts in adaptive-learner. This engine change unblocks adaptive-learner#1707.

Closes #83

Checklist

• make release-check is green (lint + typecheck + test + build). 509 tests pass.
• Behavior/logic changes followed TDD: failing test first, then the fix (6 new tests: happy-path, edge, boundary, reproduction).
• Commits follow Conventional Commits (feat:), one commit per logical change.
• schema/*.json change is deliberate: byte baseline refreshed in the same commit, changelog entry added. No generated lesson types were affected (the manifest schema has no generated TS mirror).

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https://claude.ai/code/session_01EK4jFLAbQN1RkEbAcVHBPz

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