Reframe Day 14 notes to Dijkstra single-source shortest-path

[Nullkernel]

Motivation

• The file mixed a spanning-tree heading with Dijkstra-specific content, which could mislead learners about the lesson focus.

Description

• Renamed the top-level heading to Single-Source Shortest Path (Dijkstra's Algorithm) to match the content.
• Replaced spanning-tree intro bullets with Dijkstra-specific context (non-negative weights, min-heap) and added a clear Problem Statement and note about negative weights.
• Preserved the existing step images and aligned surrounding text to shortest-path terminology.
• Changes were committed to Notes/Day-14-spanning-tree.md (commit reported 1 file changed, 13 insertions, 8 deletions).

Testing

• Inspected the updated file with nl -ba Notes/Day-14-spanning-tree.md | sed -n '1,220p' and confirmed the new headings and content were present.
• Ran git status --short and verified the change was staged/committed successfully (commit output indicated the file modification).

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Codex Task