Off-by-one / boundary error

Index/range/loop bounds are miscalculated by one, causing omission, duplication, or out-of-range access.

Clarify inclusive vs exclusive bounds; prefer length-based loops and safe slicing; add boundary tests and fuzzers

Observable signs that may indicate Off-by-one / boundary error:

• First/last element skipped or duplicated.
• Index out-of-range on boundary inputs.
• Allocated size differs by exactly one element.