perf(graph): cache the visit order of graphs

[marco-c]

Graphs are immutable, but visit_postorder and visit_preorder sorted them topologically again every time they were called. The full task graph is visited once per registered verification (11 times in taskgraph alone), then again to serialize it. The target task graph is visited several times during optimization. Now the order is computed once per graph and direction, and cached like links_and_reverse_links_dict already is.

Also, during optimization:

• index paths are gathered by iterating over the tasks directly, as the order doesn't matter;
• remove_tasks uses the cached reverse links instead of building them again.

On a synthetic graph of 20,210 tasks and 40,200 edges, verifying the full task graph takes 0.34s instead of 0.73s, and optimizing it takes 1.57s instead of 2.19s.

[codspeed]

Merging this PR will not alter performance

✅ 24 untouched benchmarks
🆕 4 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	test_for_each_task_repeated[btree]	N/A	668.1 ms	N/A
🆕	test_for_each_task_repeated[diamond]	N/A	3 s	N/A
🆕	test_for_each_task_repeated[fan]	N/A	697.4 ms	N/A
🆕	test_for_each_task_repeated[linear]	N/A	665.2 ms	N/A

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_{Comparing marco-c:cache_visit_order (2894a55) with main (c6f48bb)}

[marco-c]

The codspeed results were overstating things because the cache was not cleared during those tests.