Parse a human duration string ("10m", "90s", "1h30m") into milliseconds.

Used by the :time_budget option (--time-budget) — a wall-clock budget for the per-mutant phase (see Mutare.Runner). The grammar is deliberately tiny: one or more <integer><unit> segments, units h/m/s, in descending order, each optional but at least one required. Any non-negative integer is allowed in a segment (so "90s" and "1h90m" are fine — there is no 0–59 range check), and the total must be positive.

A bare number with no unit ("600") is rejected — the unit would be ambiguous — as is any other malformed input (wrong order, an unknown unit, a unit with no number). The parser never guesses; callers get an {:error, reason} they can surface verbatim.

iex> Mutare.Duration.parse("1h30m")
{:ok, 5_400_000}

iex> Mutare.Duration.parse("90s")
{:ok, 90_000}

iex> match?({:error, _}, Mutare.Duration.parse("600"))
true

iex> match?({:error, _}, Mutare.Duration.parse("30s10m"))
true