feat: add v0.3 normalised comparison reasoning

Add explicit reasoning guidance for normalising counts by exposure/denominator,
distinguishing total count from rate, and avoiding correlation-as-causation errors.

Changes:
- prompts/reconstruct-v0.3.md: new prompt with normalisation discipline
- lib/reconstruction/prompt.js: v0.3 loader + env var override support
- lib/analysis.js: defer DEFAULT_PROMPT_VERSION to prompt module (defaults to v0.3)
- PROMPT_VERSIONS extended to [v0.1, v0.2, v0.3]
- tests/v03-reasoning.test.js: 34 focused tests covering prompt loading, schema validation, guidance completeness, and target scenario fixture
- playwright.config.js + tests/smoke.test.js: minimal UI smoke test for browser rendering
- package.json: add @playwright/test as devDependency

Default switches to v0.3; v0.2 selectable via promptVersion or RECONSTRUCTION_PROMPT_VERSION env var.

playwright.config.js

@@ -0,0 +1,5 @@
+import { defineConfig } from "@playwright/test";
+export default defineConfig({
+  use: { headless: true, screenshot: "only-on-failure", actionTimeout: 120000 },
+  testMatch: "**/tests/smoke.test.js",
+});