fix(#1905): normalize hand-authored backstop marker so it can't degrade to green (#1909)

* fix(#1905): normalize hand-authored backstop marker so it can't degrade to green

A hand-authored non-inferable `backstop` truth with a stray trailing space (or
surrounding quotes) silently graded {status:green} instead of abstaining — the exact
#1154 false-pass. `truthVerification` returned null for any value != 'backstop'/'explicit',
and the frontmatter continuation-KV parser preserved the stray whitespace captured inside
the quotes. Normalize the marker before comparison (Postel) AND trim the continuation-KV
value at the parser (durable root cause; also cleans the sibling check_target path).

Regression: a trailing-space/quoted backstop truth now abstains (insufficient_spec),
end-to-end from a hand-authored must_haves.truths block (#1820 rail).

Refs #1905, epic #1904.

* chore(changeset): Fixed fragment for #1909 (backstop marker normalize)

src/frontmatter.cts

@@ -482,7 +482,11 @@ function parseMustHavesBlock(content: string, blockName: string): unknown[] {
       } else {
         const kvMatch = trimmed.match(/^(\w+):\s*"?([^"]*)"?\s*$/);
         if (kvMatch) {
-          const val = kvMatch[2];
+          // Trim: a quoted value like `"backstop "` captures the inner trailing space in group 2.
+          // Left untrimmed, a hand-authored `must_haves` marker degrades (a `backstop` truth silently
+          // grades green instead of abstaining — #1905, the #1154 false-pass; also the sibling
+          // check_target/violationFixture path). Whitespace is never semantic in a scalar KV value.
+          const val = kvMatch[2].trim();
           // Try to parse as number
           (current)[kvMatch[1]] = /^\d+$/.test(val) ? parseInt(val, 10) : val;
         }