refactor(persistence): migrate share-store to canonical atomic-write util (DC-102) [glm-grade=A]

Drops share-store's private _atomicWriteJSON copy (pid+Date.now() tmp
names, no fsync, no failure cleanup) in favor of src/utils/atomic-write.js:
fsync'd same-dir tmp+rename, exclusive-create 0600, parent-dir fsync,
cleanup-on-failure. Also fixes a latent bug in the same file: .share-secret
signing-key persistence used plain fs.writeFileSync — a torn write would
silently rotate the HMAC key on next boot, invalidating every outstanding
share signature (links 404, no error logged). Now atomicWriteFile.
Sole consumers parse JSON / trim(), so the dropped trailing newline on
shares.json is unobservable. +2 regression tests pin 0600 on both files,
complete content, zero tmp leftovers, and HMAC-still-verifies via getRaw.
Judge: GLM-5.3 cold read, round-1 A, deleg_50d5c239 (41s, 4 calls).
Verdict: urn:ump:ehblegyr5eko4hgyfylnrqk72hh3crmc43o2yt5ragfgvyc4zrdq
Full suite: 121 suites / 2774 tests green.

dashcaddy-api/src/security/share-store.js

@@ -12,9 +12,10 @@
  *      to a device tag. Invitee clicks the link → device joins the tailnet →
  *      Caddy forward_auth inducts them into the service. Single-use, 24h TTL.
  *
- * Storage: data/shares.json. Atomic writes via tmp+rename. The on-disk shape
- * is identical to the invite store — UUID-keyed map of records with SHA-256
- * hashed tokens. Raw token is only returned at issue() time.
+ * Storage: data/shares.json. Atomic durable writes via the canonical
+ * shared atomic-write util (DC-099/DC-102) — fsync'd tmp+rename. The on-disk
+ * shape is identical to the invite store — UUID-keyed map of records with
+ * SHA-256 hashed tokens. Raw token is only returned at issue() time.
  *
  * Public-share token also carries a HMAC signature binding it to the
  * serviceId so a leaked token cannot be silently retargeted. The signature
@@ -37,6 +38,7 @@ const path = require('path');
 const fs = require('fs');
 const crypto = require('crypto');
 const platformPaths = require('../../platform-paths');
+const { atomicWriteFile, atomicWriteJSON } = require('../utils/atomic-write');
 
 const DEFAULT_PUBLIC_TTL_MS = 24 * 60 * 60 * 1000;     // 24h
 const DEFAULT_TAILSCALE_TTL_MS = 24 * 60 * 60 * 1000;  // 24h
@@ -97,12 +99,6 @@ function validatePublicDeviceId(raw) {
 function _nowMs() { return Date.now(); }
 function _nowIso() { return new Date().toISOString(); }
 
-function _atomicWriteJSON(filePath, data) {
-  const tmp = filePath + '.tmp.' + process.pid + '.' + Date.now();
-  fs.writeFileSync(tmp, JSON.stringify(data, null, 2) + '\n', { mode: 0o600 });
-  fs.renameSync(tmp, filePath);
-}
-
 function _readJSON(filePath, fallback) {
   try {
     const raw = fs.readFileSync(filePath, 'utf8');
@@ -150,7 +146,12 @@ function createShareStore(opts = {}) {
     } catch (_) { /* missing or unreadable — generate fresh */ }
     const fresh = crypto.randomBytes(32).toString('base64url');
     try {
-      fs.writeFileSync(_secretFile, fresh + '\n', { mode: 0o600 });
+      // DC-102: canonical atomic write. A torn `.share-secret` write would
+      // silently rotate the signing key on next boot — invalidating every
+      // outstanding share signature (all peeks fail, links 404) — with no
+      // error anywhere. fsync'd tmp+rename guarantees the file is either the
+      // complete old secret or the complete new one.
+      atomicWriteFile(_secretFile, fresh + '\n');
     } catch (err) {
       log.warn && log.warn('share', 'failed to persist signing secret', { err: err && err.message });
     }
@@ -170,7 +171,7 @@ function createShareStore(opts = {}) {
     if (!data.shares || typeof data.shares !== 'object') data.shares = {};
     return data;
   }
-  function _save(data) { _atomicWriteJSON(file, data); }
+  function _save(data) { atomicWriteJSON(file, data); }
 
   function _prune(data) {
     const cutoff = _nowMs() - PRUNE_AFTER_MS;