fix(webllm): cleanRewriteLine should strip inline **verb** bold delimiters from rewrite output

[Vaishnavi1709]

Surfaced by

The eval harness from #65 (PR #149). Gemma 2 (2B) under the terse prompt variant emits per-bullet outputs that bold just the leading verb in markdown, e.g.:

**Increased** weekly active users by 1500% (120,000 to 1.8 million)…
**Spearheaded** a growth team of six individuals…
**Streamlined** the checkout process…

Example from tests/fixtures/rewrite/reports/eval-rewrite-gemma-2-2b-it-q4f16-1-mlc-…json:

{
  "text": "**Triaged** an average of 200+ inbound customer support tickets per week…",
  "startsWithActionVerb": false
}

The **Triaged** token survives into the bullet text. The first-token check in verbs.ts::startsWithActionVerb strips non-[a-z] chars after lowercasing, so the verb match itself still works — but the visible bullet contains literal ** delimiters that a user would see if this output were ever surfaced in product.

Root cause

src/lib/webllm/post-process.ts::cleanRewriteLine strips markdown emphasis only when the whole bullet is wrapped:

const withoutEmphasis = withoutPrefix
  .replace(/^\*\*(.+)\*\*$/s, "$1")
  .replace(/^\*(.+)\*$/s, "$1")
  .replace(/^_(.+)_$/s, "$1");

Inline **verb** at the start of a longer bullet doesn't match ^\*\*(.+)\*\*$, so the delimiters pass through.

Fix

Add a narrow pre-strip that removes **word** only when it leads the bullet (a verb-bolding pattern), without touching legitimate mid-line emphasis:

const withoutLeadingBold = trimmed.replace(/^\*\*([A-Za-z][\w-]*)\*\*\s+/, "$1 ");

Match characteristics:

• ^\*\* — must be at the start
• ([A-Za-z][\w-]*) — captured word (single token, alphanumeric/hyphen)
• \*\*\s+ — closing delimiter followed by at least one space (so we only match when there's body text after)

Apply before the existing whole-bullet emphasis strip so the order is: leading-verb-bold strip → whole-bullet emphasis strip → list-marker strip → quote strip.

Add unit tests in post-process.test.ts:

expect(cleanRewriteLine("**Increased** weekly active users by 1500%."))
  .toBe("Increased weekly active users by 1500%.");
expect(cleanRewriteLine("**Streamlined the** checkout"))   // not just one word
  .toBe("**Streamlined the** checkout");                    // unchanged — defensive

Tracking

• Surfaced by the Gemma report committed in PR feat(webllm): rewrite-quality eval harness (#65) #149.
• Related: Phase 3: Rewrite-quality eval harness (model + prompt comparison) #65 (the eval harness itself).