fix: allow sheet closing text in SVS recipes

[Arthur031221]

Authors who put a literal </sheet> in a Text Template prompt get SVS_TRAILING instead of rendered text. A prompt showing someone how to write SVS markup is one case.

The SVS parser found the first </sheet> in the source, even when it was inside a quoted recipe value. It now uses the final closing tag. One regression test round-trips the value through the SVS formatter. Another parses the recipe into a Text Template and renders the prompt. Both failed with the original parser and pass with the fix.

Text after the actual closing tag is still rejected. Malformed files with a duplicate closing tag remain rejected, though their error changes from SVS_TRAILING to SVS_RULE.

Checks run:

• pnpm check
• pnpm test (1,105 passed, 21 skipped, 0 failed)