Treat a C# verbatim string as literal in the comment masker

[ptr727]

strip_strings() treats a backslash as an escape inside every quoted span. In a C# verbatim string (@"...") the backslash is literal, so a verbatim string ending in one consumes its own closing quote, the masker never leaves the string, and the rest of the line is blanked:

LINE : var p = @"C:\tmp\"; // Two things. Here.
MASK : var p = @"
EXTR : []

The trailing comment is invisible to extracted_comments(), so every comment rule goes blind on that line.

Scope

• False negatives only. A comment goes unseen; no code line is reported as prose. That is the opposite direction from the defect fixed in Bound block-comment scanning by the earliest line-comment marker #461.
• Line-scoped. strip_strings() resets its quote state per line, so a broken line cannot corrupt the lines below it.
• Warn-only today. comment-wrap and comment-case report without gating, and charset plus dupword are what gate, so nothing fails CI because of this.
• C# only. No other fleet syntax has a verbatim-string form.

The related half

The verbatim escape for a quote is a doubled "", not \". The masker reads the first as closing and the second as reopening, so @"say ""hi"" now" is mis-paired even without a trailing backslash.

Fix sketch

Recognize the @" opener and mask to the next unpaired ", treating "" as a literal quote and the backslash as an ordinary character. The syntax table would need to say that C-like covers a language with verbatim strings, since the masker is currently language-agnostic within C_LIKE and .js, .ts, .json, and .jsonc share that entry while having no verbatim form.

Wanting a case in test_prose_lint.py covering @"C:\tmp\" with a trailing comment, watched failing first.

Found by Copilot in the low-confidence block of its review of promotion PR #460, and deferred there rather than expanding that promotion.

[ptr727]

Fixed in #464, squashed to develop as 7bf8a12.

Both halves are covered: a verbatim string ending in a backslash keeps its closing quote, and a doubled quote is read as one escaped character rather than a close and a reopen. C# took its own syntax entry for it, since .c, .cpp, .js, .ts, .json, and .jsonc share the C-like markers but have no verbatim form.

Two further cases came out of the review of that PR and are fixed with it: the interpolated spellings @$" and $@" are both recognized, and verbatim rules are restricted to the double-quoted form so @'a''b' is read as an ordinary char literal.

Closing by hand rather than by keyword, since the fix landed on develop and Closes only fires on a merge into the default branch. It reaches main with promotion #460.