fix(nesting-guard): export dataDir from src/config/paths; harden fallback to platform-paths (DC-077) [glm-grade=B]

Pre-fix, every dashcaddy-api container startup logged:
  [nesting-guard] Skipped: The "path" argument must be of type string. Received undefined
because src/utilities/nesting-guard.js does require('../config/paths') and
calls paths.dataDir — but src/config/paths.js imported platformPaths and
only re-exported its specific files (SERVICES_FILE, CONFIG_FILE, etc);
dataDir was never re-exported, so paths.dataDir was undefined.

Result: path.join(undefined, 'data') threw TypeError, the outer try/catch
swallowed it, and the entire nesting-guard became a silent no-op. The
cleanup that prevents recursive data/data/data/... directory duplicates
never ran on any startup. Bug class is 'silent functional no-op' (same
family as DC-056 AggregateError visibility).

(1) src/config/paths.js (+11): re-export dataDir as
SERVICES_DIR-derived (with platformPaths.dataDir fallback). dataDir is
the dirname of SERVICES_FILE in container (env override wins), which
equals /app/data — same value platform-paths.dataDir computes for the
default config. Either path is fine; SERVICES_DIR is preferred because it
respects env-override.

(2) src/utilities/nesting-guard.js (+13/-2): defensive fallback to
require('../../platform-paths').dataDir if paths.dataDir is missing
(any future export-shape drift or older caller). Explicit skip-warn
instead of silent catch when both paths fail.

(3) __tests__/nesting-guard.test.js (NEW, 112 lines, 4/4 passing):
isolates module cache per test, exercises (a) cleanup when nested
data/data exists, (b) no-op when clean, (c) dataDir export contract,
(d) dataDir === dirname(SERVICES_FILE) under env override. No jest.doMock
leaks across tests (verified via 4-call probe sequence).

Verified: 4/4 tests passing. Full repo suite: 100/104 suites / 2335/2335
tests passing (4 pre-existing failures in __tests__/billing/* are
unrelated module-resolution issues in src/billing/invoice.js, confirmed
unaffected by this change via stash+rerun).

GLM-5.3 round 1: B (ship, one polish nit — trailing newline on test
file, folded in same commit per multi-round-fix-first protocol).

Deploy plan: container rebuild + atomic swap via /opt/dashcaddy/start.sh
on DNS2; live-verify status.sami=200, dashcaddy-api=Up+healthy, and
absence of [nesting-guard] Skipped log line in container logs.

dashcaddy-api/src/config/paths.js

@@ -31,6 +31,17 @@ module.exports = {
   CADDY_ADMIN_URL,
   SERVICES_FILE,
   SERVICES_DIR,
+  // Re-export the resolved data directory so other modules (notably
+  // src/utilities/nesting-guard.js) can locate `/app/data` without having to
+  // also require('../../platform-paths') — keeps a single source of truth for
+  // the data dir on the src/config/paths surface. Without this, `dataDir`
+  // resolves to `undefined`, and `path.join(undefined, 'data')` throws
+  // `TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type
+  // string. Received undefined` at startup (DC-077 fingerprint). Fall back to
+  // platformPaths.dataDir if SERVICES_DIR is somehow not a string (defensive —
+  // SERVICES_DIR is computed from a path.dirname() of a string so it always
+  // is, but the cost of guarding is one branch).
+  dataDir: typeof SERVICES_DIR === 'string' && SERVICES_DIR ? SERVICES_DIR : platformPaths.dataDir,
   CONFIG_FILE,
   DNS_CREDENTIALS_FILE,
   TAILSCALE_CONFIG_FILE,

dashcaddy-api/src/utilities/nesting-guard.js

@@ -14,8 +14,19 @@ const path = require('path');
 module.exports = function nestingGuard() {
   try {
     const paths = require('../config/paths');
-    const dataDir = paths.dataDir;
-    const dataDataPath = path.join(dataDir, 'data');
+    const dataDir = paths && paths.dataDir;
+    // Defensive: if paths.dataDir is undefined (older callers or a future
+    // export-shape drift), fall back to platformPaths.dataDir directly so the
+    // guard can still execute. Pre-fix this branch was swallowed silently by
+    // the outer try/catch, leaving the entire nesting-guard a no-op (DC-077).
+    const effectiveDataDir = typeof dataDir === 'string' && dataDir
+      ? dataDir
+      : require('../../platform-paths').dataDir;
+    if (typeof effectiveDataDir !== 'string' || !effectiveDataDir) {
+      console.warn('[nesting-guard] Skipped: dataDir unavailable from src/config/paths and platform-paths');
+      return;
+    }
+    const dataDataPath = path.join(effectiveDataDir, 'data');
 
     // If data/data exists, it's a recursive duplicate — remove it
     if (fs.existsSync(dataDataPath)) {