feat(storable): implement Storable for unbounded 2-tuples [v0.6 backport]

[lpahlavi]

Cherry-pick of #434 onto v0.6.9.

Summary

• Replaces the two todo!() panics in the (A, B) Storable impl (to_bytes and from_bytes) with a working implementation for unbounded tuples.
• The bounded 2-tuple wire format is unchanged — no backwards-compatibility break.
• Adapted from the main cherry-pick: because v0.6 has no into_bytes method, the unbounded logic is inlined directly in to_bytes, matching the style of the v0.6 3-tuple implementation.

Wire format for unbounded (A, B)

If A is fixed-size:
  <a_bytes> <b_bytes>
Otherwise:
  <size_lengths (1B)> <size_a (1-4B)> <a_bytes> <b_bytes>

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Backports the main-branch work from #434 to v0.6.9, implementing support for (A, B) tuple serialization/deserialization when either element is unbounded, while keeping the existing bounded 2-tuple wire format unchanged for backwards compatibility.

Changes:

• Implement unbounded (A, B) tuple Storable::to_bytes and Storable::from_bytes using the established size-lengths header approach (only when needed).
• Add property-based and unit tests covering unbounded 2-tuples, mixed bounded/unbounded tuples, optional tuples, and BOUND/fixed-size expectations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/storable/tuples.rs	Replaces todo!() paths with a concrete unbounded 2-tuple wire format implementation consistent with existing tuple encoding patterns.
src/storable/tests.rs	Adds proptest coverage and bound assertions to validate unbounded and mixed bounded/unbounded 2-tuple behavior.

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