fix(validation): unique id per question for multi-question steps (#2514)

[jung-thomas]

Fixes #2514.

Problem

An author can put several [VALIDATE_N] blocks in a single step (the number keys the step, not the question). The parser already accumulates them into one array and Validation.vue already renders N questions per step — but every question was assigned the same id validate-${stepNum} (scripts/parsers/rules.ts:277,301).

Because the client keys answer state and per-question verdicts by q.id, three questions with identical ids collided onto one slot:

• couldn't validate the questions (shared answer + all-or-nothing grader failed the step),
• MCQ selections appeared stuck/unchangeable,
• the parser's own sibling maps (${stepNum}:${q.id}) overwrote each other, collapsing AI-graded specs to one row per step.

Fix

Thread an ordinal (the count already in the step's array) into parseBlock and assign:

• first question in a step → validate-N (unchanged — single-question steps and all published content are byte-for-byte identical),
• each subsequent question → validate-N-<ordinal>.

The id and the sibling-map keys are computed in one place so they stay consistent. No frontend change needed — Validation.vue/grading.ts already key by id.

Tests

New scripts/parsers/__tests__/rules-multi-question-per-step.test.ts:

• three blocks accumulate into one step,
• distinct id per question + first keeps legacy validate-N,
• single-question step regression,
• sibling map keeps a distinct entry per question (no AI-graded overwrite).

Full parser + validation suites green (227 tests): npx vitest run scripts/parsers/ hugo-apps/src/validation/.