pxf: the parser drops a trailing comment on an entry's line, so formatting moves or loses it #193

Found while porting the formatter fix (#188, PR #192). It was measured on main and is not related to directives.

What happens. The Java parser never attaches a comment that ends an entry's line to that entry. b = 1 # tc formats as b = 1. When another entry follows, # tc is written on the line above it, as that entry's leading comment. At the end of a block the comment is gone. protowire-go keeps it as the entry's trailing comment and writes it back in place.

Why it matters. pxf fmt on a Java build changes what a document's comments describe, or deletes them. It is the one difference between Java's and Go's formatter output in PR #192's byte comparison (13 of 14 documents identical, the 14th differs only here). The C# port has the same bug, trendvidia/protowire-csharp#108.

Done when: a comment after an entry on the same line is kept as that entry's trailing comment, both at the end of a block and in the middle of one, and formatting writes it back on the same line, byte-identical to protowire-go's FormatDocument. Each case has a test.