Add public Verify API and Fix operation

[aalpar]

Summary

• Add Verify to both API surfaces — Verify(h heap.Interface) bool (v1) and (*Deheap[T]).Verify() bool (generic) — for checking the min-max heap invariant in O(n)
• Add Fix to both API surfaces for re-establishing heap order after mutating an element at a known index in O(log n)
• Improve Init/From to use Floyd's bottom-up construction with a valid-heap fast path
• Replace the test-only isHeap helper with Verify assertions across all unit, randomized, and fuzz tests
• Add testable examples for Verify on both API surfaces

Test plan

• go test ./... passes
• Verify assertions added to all test and fuzz functions
• Testable examples for both ExampleVerify and ExampleDeheap_Verify

[Copilot]

Pull request overview

This PR adds public heap validation and repair primitives (Verify and Fix) across both the v1 (heap.Interface) and generic (Deheap[T]) APIs, and updates heap construction (Init / From) to use a Floyd bottom-up build with a fast-path when the input is already valid.

Changes:

• Add Verify (O(n)) and Fix (O(log n)) to both API surfaces.
• Update Init / From to skip work when already valid, otherwise build via Floyd bottom-up heapification.
• Replace the test-only isHeap helper with Verify assertions; add new unit/fuzz coverage and examples for Verify.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
deheap.go	Adds v1 Fix and Verify, plus valid helper; updates Init to use validity fast-path + Floyd build.
ordered.go	Updates From to Floyd build with validity fast-path; adds generic Fix, Verify, and orderedValid.
deheap_test.go	Replaces isHeap usage with Verify; adds Fix unit/random/fuzz coverage; removes isHeap helper.
ordered_test.go	Adds Verify assertions broadly; adds Fix unit/random/fuzz coverage for the generic heap.
example_intheap_test.go	Adds a testable example for v1 Verify.
example_ordered_test.go	Adds a testable example for generic (*Deheap[T]).Verify().

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[Copilot]

In Fix's cross-level check after swapping with a grandchild, the condition h.Less(p, v) == min is asymmetric: when min == false it becomes true for both p > v and p == v (because Less is strict), causing unnecessary swaps/bubbleups on equal values and making the intent harder to reason about. Consider spelling this out explicitly (min-level: swap if parent < child; max-level: swap if child < parent), matching the generic ordered.go implementation and avoiding equality-triggered work.

Suggested change

	if h.Less(p, v) == min {
	if (min && h.Less(p, v)) || (!min && h.Less(v, p)) {

[Copilot]

Fix is a public method and (like Remove) will panic if i is out of bounds for non-empty heaps. Consider documenting the bounds requirement (and panic behavior) in the comment so callers know i must be in [0, Len()).

Suggested change

	//
	//
	// The index i must be in the range [0, d.Len()).
	// It panics if i is out of bounds.
	//