Type-annotation drift: size/count fields use `DollarDecimal` instead of `FixedPointCount`

[TexasCoding]

From Wave 5 architecture + WS audits — F-N-02, F-N-09, F-N-11, F-N-12, F-P-07. Severity: medium (silent-bug risk).

What's wrong

Several size/volume/count fields are annotated as `DollarDecimal` even though their wire alias is `_fp` (FixedPointCount) and the inline comments explicitly say "FixedPointCount":

• `kalshi/models/markets.py:83-111` — `Market.yes_bid_size`, `yes_ask_size`, `no_bid_size`, `no_ask_size`, `volume`, `volume_24h`, `open_interest`
• `kalshi/models/markets.py:220-227` — `Candlestick.volume`, `open_interest`
• `kalshi/models/orders.py:111` — `Fill.count` (while sibling `Order.count` at line 56 is correctly `FixedPointCount`)

`DollarDecimal` strongly implies a price — misleading.

Why it works today (and the latent bug)

`_to_decimal_dollars` (`kalshi/types.py:13-27`) and `_to_decimal_fp` (`:47-60`) are byte-identical implementations. If one parser ever diverges (a per-family scale factor, different precision rule), every mistyped size/volume/count silently breaks.

Related fix

`OrderbookDeltaPayload.side: str` at `kalshi/ws/models/orderbook_delta.py:48` — should be `Literal["yes", "no"]`. The orderbook manager dispatches on `side == "yes"` vs anything-else (`orderbook.py:72`); any future server bug emitting `"YES"` or `"both"` silently routes to the `no` side.

Fix

• Change the mistyped fields to `FixedPointCount`.
• Tighten `OrderbookDeltaPayload.side` to `Literal["yes", "no"]`.
• Consolidate `_to_decimal_dollars` and `_to_decimal_fp` to a single shared callable (and reference it from both `Annotated` aliases) — keeps the type distinction at the annotation layer, removes the copy-paste hazard. Add a comment noting the intentional same-implementation choice.