perf: replace O(n²) concat with push in kalshi fetcher

[realfishsam]

Fixes #343

[realfishsam]

PR Review: VERIFIED

What This Does

Replaces three allEvents = allEvents.concat(events) calls with allEvents.push(...events) in the Kalshi fetcher's paginated event-fetching loops. Both produce the same result, but push mutates the existing array in-place instead of allocating a new copy on every page.

Blast Radius

Minimal. Three lines changed in core/src/exchanges/kalshi/fetcher.ts. The call sites (fetchEventsForMarkets, fetchAllWithStatus, fetchPageWithStatus) are internal pagination loops where allEvents is a local let array -- the old code was already mutating the binding via reassignment, so switching to push is semantically equivalent and avoids O(n) copies per page.

Findings

• No issues found. The change is correct and safe. push(...spread) is the standard pattern for accumulating into a local array.
• One edge-case to be aware of: if events ever contained tens of thousands of elements in a single page, push(...events) could hit the call-stack argument limit (~65k). Given BATCH_SIZE governs page size, this is not a realistic concern here.

Semver Impact

patch -- internal perf improvement, no API surface change.

Risk

Low.