Fix heap free-list ordering when freeing the highest-address block

[LinusU]

A current bug that ChatGPT 6 Pro noticed when reviewing some other code. The code in this PR was produced by DeepSeek v4 Flash 0731, running locally on my computer 🎉

When a freed region belongs after every existing free-list node, the insertion search left insert_index at 0, so the region was inserted at the front of the list, breaking ascending-address ordering and preventing coalescing with the preceding neighbor. Adjacent free regions could then remain unmerged, causing allocations to fail despite sufficient contiguous free memory.

Initialize the fallback insertion position to nodes.len() so appending at the end works. Adds regression tests covering the deterministic A/C/B free order, all six freeing permutations with invariant checks, and repeated-free behavior.