evals: split touchfiles into pure data + selection logic + facade

touchfiles.ts listed ITSELF in GLOBAL_TOUCHFILES, so adding one test's
dep entry forced the full ~$38 / 30-45min suite — measured on 21.9% of
recent commits (42/192). The self-reference existed because data and
logic shared a file: any edit COULD be a selection-logic change.

Now: touchfiles-data.ts (the four maps, literals only, zero imports —
the future map-diff target), test-selection.ts (matchGlob/detectBase
Branch/getChangedFiles/selectTests), and touchfiles.ts as a re-export
facade so all ~12 import sites are untouched. GLOBAL_TOUCHFILES drops
the self-ref, adds test-selection.ts (logic stays maximally
conservative), and TEMPORARILY adds touchfiles-data.ts until the
map-diff change lands. New free test pins the literal-only property
(comment-aware state-machine scan with a self-test) and facade export
parity (===), so neither can silently rot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/helpers/test-selection.ts

@@ -0,0 +1,94 @@
+/**
+ * Diff-based test selection for E2E and LLM-judge evals — the LOGIC half.
+ *
+ * Each test declares which source files it depends on ("touchfiles") in
+ * ./touchfiles-data.ts (literals only — see the note there). The test runner
+ * checks `git diff` and only runs tests whose dependencies were modified.
+ * Override with EVALS_ALL=1 to run everything.
+ *
+ * Import sites should keep using the ./touchfiles facade, which re-exports
+ * both this module and the data module.
+ */
+
+import { spawnSync } from 'child_process';
+
+import { GLOBAL_TOUCHFILES } from './touchfiles-data';
+
+// --- Glob matching ---
+
+/**
+ * Match a file path against a glob pattern.
+ * Supports:
+ *   ** — match any number of path segments
+ *   *  — match within a single segment (no /)
+ */
+export function matchGlob(file: string, pattern: string): boolean {
+  const regexStr = pattern
+    .replace(/\./g, '\\.')
+    .replace(/\*\*/g, '{{GLOBSTAR}}')
+    .replace(/\*/g, '[^/]*')
+    .replace(/\{\{GLOBSTAR\}\}/g, '.*');
+  return new RegExp(`^${regexStr}$`).test(file);
+}
+
+// --- Base branch detection ---
+
+/**
+ * Detect the base branch by trying refs in order.
+ * Returns the first valid ref, or null if none found.
+ */
+export function detectBaseBranch(cwd: string): string | null {
+  for (const ref of ['origin/main', 'origin/master', 'main', 'master']) {
+    const result = spawnSync('git', ['rev-parse', '--verify', ref], {
+      cwd, stdio: 'pipe', timeout: 3000,
+    });
+    if (result.status === 0) return ref;
+  }
+  return null;
+}
+
+/**
+ * Get list of files changed between base branch and HEAD.
+ */
+export function getChangedFiles(baseBranch: string, cwd: string): string[] {
+  const result = spawnSync('git', ['diff', '--name-only', `${baseBranch}...HEAD`], {
+    cwd, stdio: 'pipe', timeout: 5000,
+  });
+  if (result.status !== 0) return [];
+  return result.stdout.toString().trim().split('\n').filter(Boolean);
+}
+
+// --- Test selection ---
+
+/**
+ * Select tests to run based on changed files.
+ *
+ * Algorithm:
+ * 1. If any changed file matches a global touchfile → run ALL tests
+ * 2. Otherwise, for each test, check if any changed file matches its patterns
+ * 3. Return selected + skipped lists with reason
+ */
+export function selectTests(
+  changedFiles: string[],
+  touchfiles: Record<string, string[]>,
+  globalTouchfiles: string[] = GLOBAL_TOUCHFILES,
+): { selected: string[]; skipped: string[]; reason: string } {
+  const allTestNames = Object.keys(touchfiles);
+
+  // Global touchfile hit → run all
+  for (const file of changedFiles) {
+    if (globalTouchfiles.some(g => matchGlob(file, g))) {
+      return { selected: allTestNames, skipped: [], reason: `global: ${file}` };
+    }
+  }
+
+  // Per-test matching
+  const selected: string[] = [];
+  const skipped: string[] = [];
+  for (const [testName, patterns] of Object.entries(touchfiles)) {
+    const hit = changedFiles.some(f => patterns.some(p => matchGlob(f, p)));
+    (hit ? selected : skipped).push(testName);
+  }
+
+  return { selected, skipped, reason: 'diff' };
+}