feat(helper): implement IsRising utility

[SAY-5]

Description

Implements the IsRising helper function as specified in #369.

Returns 1 if the current value is strictly greater than the value n periods ago, and 0 otherwise.

$$\text{IsRising}_t = \begin{cases} 1 & \text{if } P_t > P_{t-n} \ 0 & \text{otherwise} \end{cases}$$

Implementation

• Generic helper function in the helper package.
• Signature: func IsRising[T Number](c <-chan T, period int) <-chan T
• Reuses Duplicate, Buffered, Skip, and Operate from the existing helper primitives.
• No new dependencies.

Tests

Five test cases covering: mixed input, default period (1), all-rising, all-falling, and equal values. Coverage: 100% on is_rising.go.

--- PASS: TestIsRising (0.00s)
--- PASS: TestIsRisingDefaultPeriod (0.00s)
--- PASS: TestIsRisingAllRising (0.00s)
--- PASS: TestIsRisingNoneRising (0.00s)
--- PASS: TestIsRisingEqual (0.00s)
PASS
coverage: 100.0% (is_rising.go)

Closes #369

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