feat(marking): borderline observations carried into improve prose

[hyperpolymath]

Stacked PR (6 of 8 follow-ups). Base = `feature/composer-runtime-rubrics` (PR #56). Merge order: #51 -> #52 -> #53 -> #54 -> #55 -> #56 -> this.

Summary

The borderline flags from PR #55 used to be visual-only. Now they travel into the improve prose as a one-line `Note:` sentence, so the warning makes it into the .docx the tutor pastes the feedback into.

Sentence shape

• One component, over_rated:

  Note: the reflective learning piece rating sits noticeably above the numeric performance — please verify before pasting.

• Two+ in same direction (grouped):

  Note: the reflective learning piece and the planning and practical SEN leadership piece ratings sit noticeably above the numeric performance — please verify before pasting.

• Mixed directions:

  Note: the reflective learning piece rating sits noticeably above the numeric performance; and the academic essay rating sits noticeably below the numeric performance — please verify before pasting.

Implementation

• `build_improve/5` now receives the borderlines map.
• `borderline_note/2` groups flags by direction (`:over_rated` / `:under_rated`) and emits a single sentence (or `nil` if no borderlines).
• Inserted between the convention sentence and the killer sentence — sits right before the closer.

Tests

• 55 total, 5 new
• Single severe over_rated -> note names component, reads "noticeably above"
• under_rated direction reads "noticeably below"
• Multiple in same direction share one clause ("ratings sit")
• Mixed directions -> "; and" join
• No borderlines -> no `Note:` line

Test plan

• 55/55 via standalone elixirc + ExUnit
• Browser: enter Q1=5/25 with rating "strong" -> Generate -> improve textarea contains the Note line
• Two over_rated -> single clause with "ratings sit"
• Mix over+under -> "; and" joining

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