feat(learning): show the solution as the step's debrief once its checks pass

[OthmaneZ05]

Summary of Changes

A step's solution was reachable only through the hint ladder's two-click reveal, so a learner who passes a step never sees the explanation written for it. Once a step's checks pass, the player now shows the solution on its own, marked debrief, with no reveal control.

• StepHints gets a passed prop. When set and the step has a solution, the solution renders as the debrief; hints already revealed stay readable, unrevealed ones stay hidden, and the reveal button disappears. A solution the learner had already revealed keeps its solution marker. Before the pass nothing changes.
• RoadmapPlayer passes it from the persisted completion state or the latest validation result, so revisiting a completed step shows its debrief too.
• New i18n key learning.player.debriefLabel (en/fr).
• docs/roadmap-format.md: the solution field is now documented as the step's debrief after a pass, to be written explanation-first, recipe last.

Types of Changes

• New feature / node type addition
• Bug fix (non-breaking change resolving an issue)
• Refactoring / structural cleanup
• Documentation update

Verification & Testing

Automated Checks

• Run npm run lint successfully with no errors (0 errors; the 5 warnings pre-date this change)
• Run npm run build successfully with no compilation errors
• Run npm test successfully (all tests pass) — 371 frontend tests, including 4 new StepHints cases: debrief shown with no control after a pass, revealed hints kept under it, an already-revealed solution keeps the solution marker, a passed step without a solution renders nothing

Manual Verification

Not exercised in the browser for this PR; the behaviour is covered by the component tests above. To check by hand: play any catalogue roadmap step to green, the debrief appears under the instruction with no hint button; use Previous to return to it, it is still there; on a step that has not passed, the hint ladder and the two-click solution reveal behave as before.

Checklist

• My code follows the repository's code style and lint standards
• I have updated the documentation or instructions if necessary
• All unit and integration tests are passing

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