fix(rewrite): keep the minus sign on a line-initial negative number (#821)

src/lib/webllm/post-process.test.ts

@@ -231,10 +231,30 @@ describe("cleanRewriteLine", () => {
       ["• 4.2% conversion lift.", "4.2% conversion lift."],
       // Not marker-prefixed at all — the same branch mangled this on its own.
       ["3.5x revenue growth.", "3.5x revenue growth."],
-    ])("does not eat a decimal a marker strip uncovers: %s", (input, want) => {
+      // The dash-side twin of the same bug (#821). `-` is the one marker that
+      // is also ordinary content — a minus sign — so the zero-space branch ate
+      // the sign a `- ` strip had just uncovered. A reduction then reads as a
+      // gain, and unlike a mangled decimal the line stays wholly plausible.
+      ["- -5% churn in Q3.", "-5% churn in Q3."],
+      ["- -12% cost reduction.", "-12% cost reduction."],
+      ["• -5% churn in Q3.", "-5% churn in Q3."],
+      ["- -3.5x infra cost.", "-3.5x infra cost."],
+      // Single-pass shapes — no marker to uncover the sign, so this branch
+      // mangled them on its own, as with the bare decimal above.
+      ["-5% churn in Q3.", "-5% churn in Q3."],
+      ["-.5% weekly churn.", "-.5% weekly churn."],
+    ])("does not eat a number a marker strip uncovers: %s", (input, want) => {
       expect(cleanRewriteLine(input)).toBe(want);
     });
 
+    it("preserves a leading minus sign, for any marker", () => {
+      // Sign-preservation counterpart to the decimal property below, asserted
+      // as a property so a new marker branch has to satisfy it too.
+      for (const marker of ["-", "*", "•", "1.", "1)"]) {
+        expect(cleanRewriteLine(`${marker} -5% churn.`)).toBe("-5% churn.");
+      }
+    });
+
     it("preserves every digit of a leading decimal, for any marker", () => {
       // The number-preservation counterpart to the `not.toContain("*")`
       // property above. Asserted as a property so a new marker branch has to

src/lib/webllm/post-process.ts

@@ -106,11 +106,25 @@ const LEADING_BOLD_WORD_PATTERN = /^\*\*([A-Za-z][\w-]*)\*\*\s+/;
  *     `- 3.5x revenue growth` became `5x revenue growth`. Silent numeric
  *     corruption on the product path, in a bullet the rewrite prompt actively
  *     asks the model to quantify. Nothing needs a tight `1.Foo`.
- *   - **`-` and `•` allow `\s*`**, so a tight `-Shipped X` still normalizes.
+ *   - **`•` allows `\s*`**, so a tight `•Shipped X` still normalizes. Safe
+ *     with zero-or-more because `•` is only ever a glyph — it never carries
+ *     meaning as part of the text it precedes.
+ *   - **`-` allows `\s*` only when what follows is not a number.** `-` is the
+ *     one marker that is also an ordinary character mid-content: a minus sign.
+ *     With a bare `\s*` this branch was the dash-side twin of the numbered bug
+ *     above — pass 1 stripped `- ` from `- -5% churn`, pass 2 read the
+ *     uncovered `-` as a glyph and ate it too, shipping `5% churn`. A
+ *     reduction reads as a gain, and the line stays grammatical, so nothing
+ *     downstream looks wrong. The bare `-5% churn` shape needed only one pass
+ *     and predates the loop entirely. `(?!\.?\d)` withholds the zero-space
+ *     branch from a sign, covering `-.5%` as well as `-5%`; the `\s+` branch
+ *     is unguarded because `- 5%` is a marker plus a positive number, and a
+ *     genuine negative is written `- -5%`. Nothing needs a tight `-5`-as-marker
+ *     reading. Fix for #821.
  *   - **`*` requires `\s+`**, because `*X*` is italics and is handled by the
  *     paired-emphasis strip instead.
  */
-const LIST_MARKER_PATTERN = /^(?:\d+[.)]\s+|[•\-]\s*|\*\s+)/;
+const LIST_MARKER_PATTERN = /^(?:\d+[.)]\s+|•\s*|-(?:\s+|(?!\.?\d))|\*\s+)/;
 
 /**
  * Runaway guard for the markdown-prefix loop in `cleanRewriteLine` — not a