Side-by-side diff drops field comments on the proposed side only

[peczenyj]

Problem

In side-by-side diff mode, the current column shows trailing/doc comments after field definitions, but the proposed column omits them. This makes a pure reordering look like it also deletes the field comments.

Example:
```
current │ proposed
───────────────────────────────────────────────────┼──────────────────────────
type Layout struct { │ type Layout struct {
TypeParams string // for a generic type ("[T]")│ TypeParams string
Note string // optional caveat shown ... │ Note string
... │ ...
```

Root cause

Upstream `fieldalignment` deliberately clears each field's `Doc`/`Comment` when building the suggested-fix text (`fieldalignment.go`: `f.Comment = nil; f.Doc = nil`, TODO golang/go#20744). structalign reads `Original` from raw source (comments intact) but takes `Proposed` straight from that comment-less fix text — hence the asymmetry.

Fix

Normalize both sides identically: strip per-field comments from the original too (and keep tags only when `-tags` is set), so the diff shows only the reordering. Falls out of a shared reprint helper.