fix(fmt): preserve braces to prevent dangling-else rebinding

[stevencartavia]

forge fmt drops braces around a single-statement nested if, causing the else to rebind to the inner if and silently changing semantics.

Input:

if (a) { if (b) return 1; } else return 2;

Output:

if (a) if (b) return 1;
else return 2;

With a=true, b=false: original returns 0, formatted version returns 2.

Fix: in is_single_line_block, refuse to elide braces when the then block wraps a single if (or while, whose body has the same risk) and the surrounding if has an else.

[grandizzy]

looks good, pls check comment re including for loops

[mattsse]

Review note from local pass; leaving the concrete issue inline.

[grandizzy]

lgtm, thank you!