fix: read liquidity_dollars instead of deprecated liquidity in kalshi

[realfishsam]

Fixes #554

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Updates the Kalshi normalizer to prefer liquidity_dollars over the deprecated liquidity field for both per-market liquidity and the event-level liquidity aggregation function.

Blast Radius

• Kalshi exchange: normalizer.ts -- two changes:
  1. normalizeRawMarket: liquidity line now reads liquidity_dollars first, falls back to liquidity
  2. eventLiquidity: aggregation function updated with same precedence
• No SDK, OpenAPI, or other exchange changes

Findings

1. Vendor spec confirms both fields are deprecated -- per core/specs/kalshi/Kalshi.yaml (line 6736-6740), liquidity_dollars is marked deprecated and "will always return '0.0000'". The old liquidity field (line 6734-6735) is also deprecated and "will always return 0". Both fields are deprecated and return zero values according to the spec.
2. Type safety concern -- KalshiRawMarket in fetcher.ts (line 31) only defines liquidity?: number. There is no liquidity_dollars property. The field is accessed via the [key: string]: unknown index signature, which means there is no compile-time guarantee it exists or has the expected type. Adding liquidity_dollars?: string to the interface would be cleaner.
3. parseFloat on potentially undefined -- in normalizeRawMarket, the expression parseFloat(String(market.liquidity_dollars || market.liquidity || '0')) will work but is convoluted. String(undefined) produces "undefined", which parseFloat returns NaN for, then || 0 catches it. It works but is fragile.
4. eventLiquidity has a different pattern -- uses parseFloat(m.liquidity_dollars || m.open_interest || m.liquidity || '0') which passes potentially non-string values directly to parseFloat. Since m.open_interest and m.liquidity are typed as number | undefined, passing a number to parseFloat is fine (it gets stringified), but the mixing of types in the fallback chain is not clean.
5. No tests added -- no test coverage for the liquidity field change.

PMXT Pipeline Check

• Field propagation: N/A (liquidity is already in all layers)
• OpenAPI sync: N/A
• Type safety: ISSUE -- liquidity_dollars not in KalshiRawMarket interface

Semver Impact

patch -- internal field preference change; no public API impact unless liquidity values were previously non-zero and are now zero (or vice versa)

Risk

Both liquidity and liquidity_dollars are documented as deprecated and returning zero in the Kalshi vendor spec. This change may be moot if Kalshi has already zeroed both fields. Cannot verify without a live API call whether liquidity_dollars actually returns meaningful data despite the spec claiming it returns "0.0000".