feat(data-structures): stabilize IndexedHeap

[tomas-zijdemans]

Promotes IndexedHeap from unstable to stable. The API design was already accepted during the unstable period, so this is the mechanical part:

• Renamed unstable_indexed_heap.ts to indexed_heap.ts (test file too)
• Export path is now ./indexed-heap, and mod.ts re-exports the module
• Dropped the 21 @experimental tags and pointed all 29 JSDoc example imports at the new path

One small change beyond the rename: the NaN rejection error now names the operation that was attempted. push() and the constructor throw Cannot push into IndexedHeap: priority is NaN, set() throws Cannot set priority in IndexedHeap: priority is NaN. Before, all paths said "Cannot set priority".