Per-category supersession tracking in scoring

[asvarnon]

Problem

score_passages() filters out all passages where superseded == true, but ClassifiedPassage::superseded means "superseded in at least one category" (see classification.rs lines 67-72). A multi-category passage (e.g., both Corrective and Stateful) marked superseded due to its Stateful aspect may still be the latest representative of Corrective.

Currently, these passages are dropped entirely from scoring output.

Proposed Solutions (pick one)

1. Per-category supersession tracking — Add a superseded_categories: Vec<ImportanceCategory> field to ClassifiedPassage so scoring can drop only the superseded categories and retain the passage for its non-superseded ones.
2. Stricter superseded flag — Only set superseded = true when the passage is superseded for all of its categories.
3. Filter adjustment in scoring — Retain the passage but zero out the category_weight contribution from superseded categories.

Context

• Identified during PR feat: add importance scoring and ImportanceSegment output (#50) #69 review (Copilot reviewer comment on scoring.rs:102)
• Comment added in c79fd9a documenting the limitation
• Depends on: supersession logic in apply_supersession() (classification.rs)

Acceptance Criteria

• Multi-category passages are not incorrectly dropped when only some categories are superseded
• Existing single-category supersession behavior is preserved
• Tests cover the multi-category edge case