Rename SSSP to SPSP - correct problem classification

[RajNilkar]

Summary

This PR renames the recently merged P_SSSP problem to P_SPSP (Single Pair Shortest Path) to correctly reflect what the implementation actually solves.

The Mistake

After the P_SSSP PR was merged, I realized the implementation incorrectly used the SSSP (Single Source Shortest Path) name for what is actually a Single Pair Shortest Path (SPSP) problem. The distinction is:

• SPSP (what was implemented): Given a weighted graph G, source vertex s, and target vertex t, find the shortest path from s to t. The instance format includes both a source and a target: ({nodes}, {edges}, s, t)
• SSSP (what the name implied): Given a weighted graph G and source vertex s only, find the shortest paths from s to every other vertex in the graph. No target node. The certificate would be a full distance table, not a single path.

What Changed

Purely a rename — no logic changes whatsoever:

• Folder: P_SSSP → P_SPSP
• Namespace: API.Problems.P.P_SSSP → API.Problems.P.P_SPSP
• Class names: SSSP → SPSP, SSSPSolver → SPSPSolver, etc.
• Problem/formal definitions updated to correctly describe SPSP
• Test file and class names updated
• All existing tests pass with no changes to logic

Future Work

A separate PR will follow implementing true SSSP correctly:

• Instance format: ({nodes}, {edges}, s) — no target node
• Certificate: full distance table from s to all reachable vertices
• Verifier: validates the complete distance table
• Graph visualization: shortest path tree (all edges used in shortest paths from s)
• Table visualization: already works for SPSP, with minor modifications coming soon