Contracts: Window edge-cases — start==end, zero window, and ledger clock-skew in `is_within_window`

[joelpeace48-cell]

Context

set_window/get_window/is_within_window (campaign/src/lib.rs ~L217–L253) gate registration by time.
Edge conditions (start == end, end < start, window of zero, boundary timestamp equality, reliance on
ledger().timestamp() skew) are easy to get subtly wrong and lock users in/out.

Scope

• Define inclusive/exclusive bounds explicitly.
• Validate start <= end on set; reject degenerate windows or define their semantics.
• Test boundary timestamps exactly at start and end.

Acceptance criteria

• Documented bound semantics.
• Tests at exact boundaries and degenerate windows.

[gloskull]

Hello, I have 4 years of experience delivering solutions for projects like this, covering backend, frontend, and smart contract development. After carefully reviewing your requirements, I have a clear understanding of the task and am confident in my ability to deliver a clean, efficient, and thoroughly tested solution on time. I prioritize code quality, maintainability, and industry best practices throughout the development process. My PR will be submitted in under 10hours.