fix(traces): make generated matrix links resolve from docs/

docs/traceability.md emitted each trace's file link with the
repo-root-relative path as the href, but the file is written to docs/ —
so every one of the 2,793 links resolved to docs/src-tauri/... or
docs/src/... and 404'd, in the Gitea repo browser and on the published
mdBook site alike. The matrix is the artefact the whole TRACES system
exists to produce, and it was unnavigable.

The href now carries a ../ prefix; the visible link text stays
repo-root-relative, since that is the path a developer greps for.

This survived because the markdown generator had no test at all — the
existing suite covers counting, coverage and dangling IDs only. UT-202
now generates a link for a file that really exists, resolves the href
against docs/, and asserts the target is on disk; it fails against the
old output. Watched red before the fix, per the red-green rule.

The live-requirements counts move with the rows added in the previous
commit: UR 75 -> 76, DR 194 -> 200, total 337 -> 344.

scripts/extract-traces.ts

@@ -23,7 +23,7 @@ interface RequirementMapping {
   [reqId: string]: TraceEntry[];
 }
 
-interface TracesData {
+export interface TracesData {
   timestamp: string;
   totalFiles: number;
   totalTraces: number;
@@ -366,7 +366,34 @@ export function readDefinedRequirements(): DefinedRequirements {
   return countDefinedRequirements(fs.readFileSync(reqPath, "utf-8"));
 }
 
-function generateMarkdown(data: TracesData): string {
+/**
+ * Path prefix that turns a repo-root-relative file path into a link target that
+ * resolves from `docs/traceability.md`, where this markdown is written.
+ *
+ * The generated matrix lives one directory below the repo root, so a bare
+ * `src-tauri/src/player/mod.rs` href resolves to `docs/src-tauri/…` and 404s —
+ * in the repo browser and on the published mdBook site alike. Every file link
+ * in the matrix was dead for this reason. The *display text* stays
+ * repo-root-relative (that is the path a developer types and greps for); only
+ * the href is rewritten.
+ *
+ * TRACES: | DR-093 | UT-202
+ */
+export const MATRIX_LINK_PREFIX = "../";
+
+/**
+ * Build the ``[`path`](href#Lnn)`` link used for one trace entry in the matrix.
+ *
+ * Exported so extract-traces.test.ts can resolve a generated href against
+ * `docs/` and assert the target exists on disk.
+ *
+ * TRACES: | DR-093 | UT-202
+ */
+export function formatMatrixFileLink(file: string, line: number): string {
+  return `[\`${file}\`](${MATRIX_LINK_PREFIX}${file}#L${line})`;
+}
+
+export function generateMarkdown(data: TracesData): string {
   let md = `# Code Traceability Matrix
 
 **Generated:** ${new Date(data.timestamp).toLocaleString()}
@@ -424,7 +451,7 @@ ${data.byType.JA.join(", ")}
     md += `**Locations:** ${entries.length} file(s)\n\n`;
 
     for (const entry of entries) {
-      md += `- **File:** [\`${entry.file}\`](${entry.file}#L${entry.line})\n`;
+      md += `- **File:** ${formatMatrixFileLink(entry.file, entry.line)}\n`;
       md += `  - **Line:** ${entry.line}\n`;
       const contextPreview = entry.context.substring(0, 70);
       md += `  - **Context:** \`${contextPreview}${entry.context.length > 70 ? "..." : ""}\`\n`;