feat: Unify spark table detection and content resolution

Export isSparkTableHeader from content-registry and use it in both CLI
and frontend table parsing. Add resolveContentRef to resolve content
references consistently. Write processed registry content back to the
file index so browser mode renders identically to CLI mode.

src/cli/completions/variable-system.test.ts

@@ -47,6 +47,7 @@ import { parseInput } from "../../components/journal/command-parser";
 import {
   inspectSparkTableCsv,
   buildSparkTableCompletion,
+  isSparkTableHeader,
 } from "../content-registry";
 
 // ---------------------------------------------------------------------------
@@ -927,6 +928,32 @@ describe("parseInput", () => {
 // content-registry (spark tables)
 // ---------------------------------------------------------------------------
 
+describe("isSparkTableHeader", () => {
+  test("detects plain dice formula", () => {
+    expect(isSparkTableHeader("d6")).toBe(true);
+    expect(isSparkTableHeader("d20")).toBe(true);
+    expect(isSparkTableHeader("d100")).toBe(true);
+  });
+
+  test("detects dice count and modifiers", () => {
+    expect(isSparkTableHeader("2d6")).toBe(true);
+    expect(isSparkTableHeader("3d6+5")).toBe(true);
+    expect(isSparkTableHeader("1d8-2")).toBe(true);
+  });
+
+  test("rejects non-dice headers", () => {
+    expect(isSparkTableHeader("Name")).toBe(false);
+    expect(isSparkTableHeader("d")).toBe(false);
+    expect(isSparkTableHeader("6")).toBe(false);
+    expect(isSparkTableHeader("d6x")).toBe(false);
+    expect(isSparkTableHeader("roll")).toBe(false);
+  });
+
+  test("trims surrounding whitespace", () => {
+    expect(isSparkTableHeader("  d20  ")).toBe(true);
+  });
+});
+
 describe("inspectSparkTableCsv", () => {
   test("detects spark table from CSV headers", () => {
     const csv = `d6,Name,Description