Flashcards: prioritize highlighted note sections during generation

[shreekardittakavi]

Spec: SDR 3.2.1 — "The system shall prioritize highlighted note sections during flashcard generation." Doc: "The generation should follow the content of the notes, and focus on the highlighted sections."

Current state

Generation in api/flashcards treats note content uniformly. #7 will detect and highlight definitions and formulas; nothing yet feeds that signal back into generation, so a highlighted definition carries no more weight than a passing aside.

Scope

• Surface the detected-important-element markers from Add note analysis for key elements and multi-topic split suggestions #7 in a form the generator can consume (marked spans or an extracted list with offsets).
• Weight generation toward highlighted content: guarantee coverage of highlighted definitions and formulas before drawing on surrounding prose.
• Apply the same weighting to quiz generation, where the requirement is identical in spirit.
• Include user-applied highlights, not only auto-detected ones — an explicit highlight is the strongest available signal of what the student thinks matters.
• Degrade cleanly for notes with no highlights: fall back to current uniform behaviour rather than producing an empty deck.

Depends on

• Add note analysis for key elements and multi-topic split suggestions #7 (note analysis for key elements)
• Add Topic-Based Retrieval and Chunked Embeddings for Flashcard Generation #57 (chunked embeddings) — chunk boundaries should not split a highlighted region

Acceptance criteria

• Highlighted definitions and formulas are represented in the generated deck
• A note with highlights produces a measurably different deck than the same note without them
• User highlights are weighted at least as strongly as auto-detected ones
• Notes with no highlights still generate normally