refactor(sdk): shared @maple/sdk-core for the browser and Effect SDKs - #1167

🟢 Confidence 4/5 · likely safe to merge
_{quality 100/100 · no findings · tests covered · risk medium}

Extracts @maple/sdk-core (sampling, error filtering/rendering, status policy, browser collectors, page-wide coordination) and adapts @maple-dev/browser to it. The delta since the last review (e780797) is the linear stack-frame parser, per-batch consent checks in the offline queue, and the extracted interactionKey; all are correct and tested.

• frameUrl parses stack frames by position, replacing the backtracking FRAME_URL regex
• Offline batches carry an endpoint+key target; drain checks hasConsent() per batch
• interactionKey extracted, so events without interactionId are still spanned
• startPerf emits through an injected StartSpan sink instead of reaching for OTel

_{e780797 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}