refactor: prepare for lazy key loading

[maksymar]

This PR refactors the codebase to prepare for lazy key loading.

• Introduces a shared LazyObject structure for both keys and values
• Unifies the API with methods like get_key / get_value, key / value, and upcoming extract_key / extract_value

The refactoring does not change functionality, but introduces a minor performance regression.
This is due to switching from direct key reads to indexed access, aligning with the future lazy-loading model.

  instructions:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min -440.00K | med +954.31K | max +79.11M]
    change %: [min -0.18% | med +0.11% | max +1.81%]

The implementation is split into two parts (refactoring and actual feature) to isolate and assess performance impact in this sensitive code path.

[github-actions]

canbench 🏋 (dir: .) 04df987 2025-05-21 15:07:53 UTC

✅ ./canbench_results.yml is up to date
📦 canbench_results_benchmark.csv available in artifacts

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Summary:
  instructions:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min -440.00K | med +954.31K | max +79.11M]
    change %: [min -0.18% | med +0.11% | max +1.81%]

  heap_increase:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

  stable_memory_increase:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[Copilot]

Pull Request Overview

This PR refactors the B‑Tree node implementation to prepare for lazy key loading by introducing a new LazyObject structure and unifying the API for handling keys and values.

• Introduces LazyObject and LazyValue, replacing earlier Value handling for lazy loading.
• Unifies API methods like get_key/get_value, key/value, and readies upcoming extract_key/extract_value.
• Updates tests and iterators to account for the new API, including changes to expected types.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

File	Description
src/btreemap/node/v2.rs	Replaces Value with LazyValue in entry loading and writes for lazy operations.
src/btreemap/node/v1.rs	Updates lazy loading from Value to LazyValue and uses get_key for key ordering.
src/btreemap/node.rs	Refactors APIs to use LazyObject/LazyValue; adjusts keys(), search(), and entry methods.
src/btreemap.rs	Adjusts test assertions to expect references for keys after API changes.

Comments suppressed due to low confidence (1)

src/btreemap.rs:3274

• [nitpick] The test has been updated to use a slice reference for keys. Confirm that this change aligns with the new API design and that all parts of the codebase expecting key ownership have been appropriately refactored.

assert_eq!(root.keys(), vec![&[6; 10_000]]);

[berestovskyy]

Looks good, thanks!