Optimize: Update transitive closure algorythm.

[hneiva]

The previous implementation scanned all edges on every iteration of a fixed-point loop, resulting in O(depth * edges) complexity. For deep dependency chains this is catastrophic - a 5000-node chain required 5000 full passes over all edges.

Replace with a standard Breadth-First Search using an adjacency list built once upfront, reducing complexity to O(vertices + edges). Benchmarks on large graphs show 22x-1333x speedups depending on graph shape, with identical results verified against the original algorithm.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.01%. Comparing base (4aacfff) to head (86d36d8).

Files with missing lines	Patch %	Lines
src/taskgraph/graph.py	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #920      +/-   ##
==========================================
- Coverage   77.01%   77.01%   -0.01%     
==========================================
  Files         130      130              
  Lines       11796    11802       +6     
  Branches     1450     1454       +4     
==========================================
+ Hits         9085     9089       +4     
- Misses       2330     2331       +1     
- Partials      381      382       +1     

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