fix: guard Map.get() on candle buckets in polymarket normalizer

[realfishsam]

Fixes #321

[realfishsam]

PR Review: VERIFIED

What This Does

Guards Map.get(snappedMs) with an if (candle) check instead of using the non-null assertion buckets.get(snappedMs)! in the Polymarket normalizer's OHLCV candle-bucketing logic.

Blast Radius

Single file: core/src/exchanges/polymarket/normalizer.ts. One code path in normalizeOHLCV. The else branch is only entered when buckets.has(snappedMs) is true (the preceding if (!buckets.has(snappedMs)) sets the entry), so get() will always return a value in practice. The guard is purely defensive.

Findings

• The guard is correct and safe. If get() somehow returned undefined (impossible given the has() check above, but defensive), the price point is silently skipped. This is acceptable for OHLCV data -- a missing candle update is better than a crash.
• Mutation concern: The candle object retrieved from the Map is still mutated in-place (candle.high = ..., candle.low = ..., etc.). This is a pre-existing pattern and not introduced by this PR. Since the Map owns these objects and they are only used locally within this method before being returned as an array, in-place mutation is acceptable here.
• No non-null assertions remain in this code path.

Semver Impact

patch -- defensive guard, no behavioral change in practice.

Risk

Low.