[codex] Add indicator cache invalidation

[sdcoffey]

Summary

• add a public CacheResetter interface and ResetCacheFrom helper
• implement cache invalidation for EMA-backed indicators
• add regression coverage for recalculating after mutable series data changes

Related issues

Addresses #53.

Validation

• GOCACHE=/private/tmp/techan-gocache mise x go@1.26.3 -- go test ./...

Stacked on #62.

[sdcoffey]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reset all built-in cached indicators

When callers use this new helper with NewMMAIndicator(...), it returns false here even though modifiedMovingAverageIndicator has a resultCache and uses returnIfCached/cacheResult, so changing an underlying candle leaves previously calculated MMA values stale with no invalidation path short of rebuilding the indicator. Since resetResultCache already accepts the shared cachedIndicator interface, the helper should also handle cached indicators that don't add their own CacheResetter method.

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