Add Cell<T> utility class

[Copilot]

Ports Box<T> from Gress as Cell<T> — a generic container for a value that may or may not be set. Unlike Nullable<T>, it treats null as a valid stored value, distinguishing "no value" from "value is null".

Changes

• PowerKit/Cell.cs — internal Cell<T> with Store(T value), TryOpen(out T value) → bool, and OpenOrDefault(T defaultValue = default!) → T
• PowerKit.Tests/CellTests.cs — covers unset, set non-null, set null, and OpenOrDefault cases

var cell = new Cell<string?>();

cell.TryOpen(out _);           // false — nothing stored yet
cell.OpenOrDefault("hello");   // "hello" — falls back to default

cell.Store(null);
cell.TryOpen(out var v);       // true — null is a valid stored value
cell.OpenOrDefault("hello");   // null — stored value takes precedence

[Tyrrrz]

@copilot please fix build errors

[Copilot]

Pull request overview

Adds an internal Cell<T> utility to represent an optionally-set value where null can still be a valid stored value (distinguishing “unset” vs “set to null”), plus unit tests validating the behavior.

Changes:

• Introduce Cell<T> with Store, TryOpen, and OpenOrDefault.
• Add tests for unset, set (non-null), set (null), and OpenOrDefault fallback behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
PowerKit/Cell.cs	Adds the Cell<T> container implementation with set/open APIs.
PowerKit.Tests/CellTests.cs	Adds unit tests covering the new Cell<T> behavior.

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[Copilot]

There are two test methods with the same name/signature OpenOrDefault_Unset_Test in this file, which will cause a compile-time error. Rename the second one (the one that stores null) to a distinct name that reflects the scenario (e.g., OpenOrDefault_Null_Test).

Suggested change

	public void OpenOrDefault_Unset_Test()
	public void OpenOrDefault_Null_Test()

[Copilot]

@copilot please fix build errors

Fixed in fa82809 — there was a duplicate OpenOrDefault_Unset_Test method; the second one (testing the null-stored case) has been renamed to OpenOrDefault_Null_Test.