Fix RSI producing NaN instead of neutral 50 on flat market

[cinar]

Summary

• Rsi.ComputeWithContext computed RS = averageGain / averageLoss and then RSI = 100 - (100 / (1 + RS)). When a window has zero gains and zero losses (a flat price run — e.g. an untraded bar, or synthetic/test data), averageGain and averageLoss are both 0, so RS = 0/0 = NaN, and the resulting RSI for that bar was NaN instead of the conventional neutral value of 50.
• Replaced the DivideWithContext → IncrementBy/MultiplyBy/Pow pipeline with a single helper.OperateWithContext that zips averageGains and averageLosses pairwise and special-cases the 0/0 case to 50; the non-flat case computes the identical RS/RSI formula as before, so normal output is unchanged.
• Checked momentum/connors_rsi.go and other callers of momentum.Rsi/NewRsi — they only consume Rsi.ComputeWithContext's output channel and don't depend on its internal pipeline shape, so behavior for non-flat windows is unaffected. Streak-based RSI (used by ConnorsRsi.StreakRsi) shares the same Rsi type and therefore benefits from the same fix, since a streak can also be flat (i.e. 0) with no separate flat-window handling of its own.

Test plan

• Added TestRsiFlatMarket in momentum/rsi_test.go: a constant closing-price series long enough to produce RSI output, asserting every emitted value is exactly 50 (not NaN).
• Verified the existing CSV-fixture-based TestRsi is unaffected — momentum/testdata/rsi.csv has no flat windows, and the test still passes.
• go build ./...
• go vet ./...
• gofmt -l . (clean for the changed files; unrelated pre-existing formatting diffs exist elsewhere in the repo, outside this change's scope)
• go test ./... (all packages pass)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.69%. Comparing base (f9f68bf) to head (5d00e82).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #455      +/-   ##
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- Coverage   92.48%   91.69%   -0.79%     
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  Files         229      230       +1     
  Lines        7302     7355      +53     
==========================================
- Hits         6753     6744       -9     
- Misses        463      525      +62     
  Partials       86       86              

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