process_hs_val stripped `//…` line comments with a naïve regex,
which devoured `https://yyy.xxxxxx.com/…` inside a backtick template
— the 'properly interpolates values 2' fixture was landing with
the HS source truncated at `https:`.
New helper _strip_hs_line_comments walks char-by-char and only
strips `//` / leading-whitespace `--` when not inside `'…'`, `"…"`,
or backticks; respects `\\`-escapes inside strings.
Suite hs-upstream-core/regressions: 11/16 → 12/16.
Smoke 0-195: 163/195 → 164/195.