Add IdlePeriod() to PringsSpecialK

[cinar]

Closes #418.

What was missing

Every other indicator in this library implements IdlePeriod() int (the subject of the consistency sweep in #413) so a caller knows how many leading values to skip before the output channel starts yielding real data. PringsSpecialK didn't, even though ComputeWithContext already computes exactly this value internally:

maxIdle := p.Sma195Roc530.IdlePeriod() + p.Roc530.IdlePeriod()

used to align every other SMA(ROC(...)) branch to the slowest one. Without an exposed IdlePeriod(), a generic caller (e.g. the indicator registry in #416) has no way to know how many leading dates correspond to its warm-up — which, with the default periods, is 724.

Its own test already worked around this by reconstructing an equivalent value from unrelated standalone Sma/Roc instances rather than calling a method on the type itself — a sign the type should expose it.

Fix

// IdlePeriod is the initial period that Pring's Special K won't yield any results.
func (p *PringsSpecialK[T]) IdlePeriod() int {
	return p.Sma195Roc530.IdlePeriod() + p.Roc530.IdlePeriod()
}

TestPringsSpecialKComputeBasicOutput now calls IdlePeriod() on the actual instance instead of hand-reconstructing the number from a throwaway Sma(530)/Roc(195) pair (which happened to total the same value only because (530-1)+195 == 194+530) — it already passed before this change and still does, confirming the new method's value matches what that test independently expected all along. Added TestPringsSpecialKIdlePeriod as a direct check.

Test plan

• go test -race -cover ./momentum/... — 92.4% coverage
• go vet, staticcheck, gosec clean on momentum/ (an unrelated pre-existing revive finding in rvi.go, a file this PR doesn't touch, is untouched)
• go build ./... and the full INDICATOR_BASE test suite (asset, backtest, cmd, helper, momentum, strategy, trend, volatility, volume) pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.29%. Comparing base (46c719e) to head (f252dbe).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
- Coverage   92.31%   92.29%   -0.03%     
==========================================
  Files         229      229              
  Lines        7276     7278       +2     
==========================================
  Hits         6717     6717              
- Misses        472      474       +2     
  Partials       87       87              

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