fix: bracket a fraction only where the slash would split it

`\frac{n(n+1)}{2}` came out as `(n(n+1))/2` and the quadratic formula
as a wall of parentheses, because the test was "is this operand a
single token" rather than "would the slash change what this means".

Brackets now appear in exactly two cases: a sum, where the slash would
otherwise divide only the last term, and a denominator that is more
than one term, where `1/2a` is read two ways. `n(n+1)/2`, `√π/2` and
`mc²/√(1 − v²/c²)` need nothing, while `1 / (2a)` and
`(−b ± √(b² − 4ac)) / (2a)` keep their brackets. A script is read
through rather than counted, since a sum above the line is unambiguous
by construction.

The slash is spaced only when something beside it is bracketed, which
is where the brackets would otherwise crowd it.

README.md

@@ -275,6 +275,8 @@ Two facts about the renderer decided the design. A paragraph is a `TextEdit` wit
 
 So this is a linearisation rather than a typesetter. `\frac{a}{b}` becomes `a/b`, `\sqrt{x+1}` becomes `√(x+1)`, `\sum_{i=1}^{n}` becomes `∑ᵢ₌₁ⁿ`, and only what Unicode has no form for falls back to a tag, so `x^{a/b}` becomes `x<sup>a/b</sup>`. `$...$`, `\(...\)`, `$$...$$` or `\[...\]` on a line of its own, and a fence labelled `math`, are all read.
 
+Brackets appear only where the slash would otherwise change what the formula means. `\frac{n(n+1)}{2}` is `n(n+1)/2` and `\frac{\sqrt{\pi}}{2}` is `√π/2`, because one product either side of a slash needs nothing, while `\frac{1}{2a}` is `1 / (2a)` because `1/2a` is read two ways, and a sum is always bracketed so the slash cannot divide only its last term: `(−b ± √(b² − 4ac)) / (2a)`. The space around the slash shows up only when something beside it is bracketed, which is where the brackets would otherwise crowd it.
+
 The one guard is worth knowing about. A lone `$` pair counts as maths only when the inside looks like maths, because `x=$a$b` and `cd $A; echo $B` are not formulas, and turning them into one is the single way this feature can make an answer worse than the dollar signs it replaced. `$5 and $10` is left exactly as written.
 
 ### Custom HTML