Change VecDeque for fixed-capacity ArrayDeque in LinearRegression

[nicolad]

Why touch the buffer in LinearRegression at all?

LinearRegression (LR) keeps a sliding window whose length (period) is known a-priori.
With the current std::collections::VecDeque the queue still grows past that length: every push_back that exceeds capacity triggers a heap re-allocation and mem-copy. Those reallocations are

• wasted work — we immediately pop_front afterwards, and
• a latency spike right on the trading hot-path.

arraydeque::ArrayDeque, in contrast, is “a circular buffer with fixed capacity … [that] can be stored directly on the stack”¹.
Its capacity is a const generic baked into the type, so pushes never allocate and the window length is never exceeded.

Aspect	VecDeque	ArrayDeque<N>
Capacity	Dynamic; grows geometrically	Compile-time fixed (const N)
Allocation	Heap (may reallocate)	None (stack/static)
Push/Pop cost	Amortised O(1) but occasional mem-copy	Strict O(1)
Enforced upper bound	Caller-dependent, not guaranteed	Compile-time assert; panic if period > N
Max window length	usize::MAX (practically unbounded)	Chosen at build time (MAX_PERIOD = 16 384)
Failure mode when full	Reallocate + copy	Programmer picks Wrapping (overwrite) or Saturating (reject)

[cjdsellers]

👌 thanks @nicolad this is a good optimization.

[nicolad]