fix: decode .sqlplan files the way SSMS writes them; name the parse failure

- lib/plan-file.ts: BOM-aware decode for UTF-8, UTF-16LE/BE (with and without BOM) and UTF-32, with a NUL-density fallback for BOM-less UTF-16. The browser had a two-byte FF FE sniff only, so a UTF-16BE or BOM-less plan arrived as NUL-filled junk and the digest blamed the export.
- digestPlan now quotes fast-xml-parser's first line instead of a fixed sentence, so a truncated file, a stray entity and a wrong file stay distinguishable.
- plan-file.ts holds the decode so the XML parser stays out of the client bundle.
- tests: 6 encodings round-trip through decodePlan into a digest; a truncated plan's message is asserted to carry a reason; e2e drops a real UTF-16LE file. 36 unit, 23 e2e.
- history.spec picked the rail's first row, which is now one of the new failing-input rows; it selects the newest finished run instead.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests-e2e/history.spec.ts

@@ -23,7 +23,9 @@ test("rename sticks, delete asks first then removes", async ({ page }) => {
 
 test("reopening a stored run renders the report with no new model call", async ({ page }) => {
   await page.goto("/app");
-  await page.locator('.rail a[href^="/app/"]').first().click();
+  // The newest finished run, not the newest row: the encoding tests above leave `failed` rows on top,
+  // and only unfinished rows carry a status pill in the rail.
+  await page.locator('.rail a[href^="/app/"]:not(:has(.pill))').first().click();
   await expect(page.locator("#report")).toBeVisible({ timeout: 30_000 });
   await expect(page.getByRole("button", { name: "Analyse" })).toHaveCount(0);
   // The stored Jev verdict is server-rendered, so it is readable even if the report body had failed.