feat: add Dijkstra's Algorithm using Adjacency Matrix

[binarymind-dev]

Description

This PR adds an implementation of Dijkstra's Shortest Path Algorithm
using an Adjacency Matrix representation, without a priority queue.

Changes

• Added graph/dijkstra_adjacency_matrix.cpp
• Computes shortest distances from a single source to all vertices in O(V^2)
• Handles disconnected graphs (unreachable vertices marked with INT_MAX)
• Includes self-test with assert statements

Difference from existing implementations

• dijkstra.cpp uses an adjacency list + priority queue (better for sparse graphs)
• bidirectional_dijkstra.cpp runs a bidirectional search from both source and target
• This implementation uses an adjacency matrix + linear scan (simpler, better
  for dense graphs, no heap overhead)

[github-actions]

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