Add pa_hold(): bookmaker margin from quoted odds #2

The package converts odds (pa_american_to_decimal, pa_implied_prob in R/odds-utils.R) and removes the margin (pa_devig() in R/devig.R, which exposes the overround as an attribute), but there is no direct way to ask the everyday question: how much margin is in this market's prices? Add pa_hold().

Proposed change

1. New exported function in R/odds-utils.R:

pa_hold <- function(odds = NULL,
                    probs = NULL,
                    odds_format = c("decimal", "american")) {
  # sum(pa_implied_prob(odds)) - 1, or sum(probs) - 1
}

2. Semantics: takes the quoted odds for ALL outcomes of one market (both sides of a total, three outcomes of a soccer match) and returns a single number, the bookmaker hold, e.g. pa_hold(c(-110, -110), odds_format = "american") is about 0.0476.
3. Validation mirrors pa_devig(): exactly one of odds or probs, at least 2 outcomes, probabilities strictly in (0, 1). Reuse pa_implied_prob() for the conversion rather than duplicating it.
4. Full roxygen block in the style of the neighboring functions, with @examples covering a 2-way and a 3-way market, plus a NEWS.md entry. Regenerate NAMESPACE with roxygen so the export lands.

Acceptance criteria

• pa_hold(c(-110, -110), odds_format = "american") returns approximately 0.0476.
• pa_hold(c(2.45, 3.40, 3.10)) returns the 3-way overround, matching attr(pa_devig(c(2.45, 3.40, 3.10)), "overround") exactly.
• pa_hold(c(-110)) errors with a clear message.
• testthat tests in tests/testthat/test-odds-utils.R cover the three cases above, no network needed.
• R CMD check passes.
• No new dependencies.