T-124: unix-domain IPC server, wired into Flutter app boot

First slice of T-99 (the D-56-path-a IPC server). What this lands:

* lib/src/ipc/paths.dart rewritten — `workspaceSocketPath(root)`
  returns the per-workspace path per D-70 (FNV-1a 64-bit hash, hex,
  no crypto dep — D-70 amended in this commit to record the hash
  choice). Old `defaultSocketPath()` removed; the lone fallback in
  facade.dart kept with a clear placeholder pending T-127.
* lib/src/ipc/server.dart — IpcServer class. ServerSocket.listen
  accept loop (D-72), 0600 socket + 0700 parent (D-71), stale-node
  probe + unlink on start, refuses to clobber a live listener.
* lib/main.dart — IpcServer started after the first dispatcher is
  built and swapped on project open (workspace path changes).
  Failure logged but non-fatal so the UI still works without IPC.
* 11 server tests + 5 path tests cover socket modes, multi-conn,
  stale unlink, live-conflict, idempotent start/stop.

T-99 children downstream of T-124 (T-125 / T-126 / T-127 / T-130)
are now unblocked.

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

lib/src/ipc/paths.dart

@@ -1,21 +1,54 @@
 import 'dart:io';
 
-/// Resolve the daemon unix-socket path.
+/// Resolve the per-workspace Unix-domain socket path served by the
+/// running clide app. Per D-70:
 ///
-/// Precedence (highest first):
-///   1. `CLIDE_SOCKET_PATH` — explicit override. Used by tests that
-///      run multiple daemons in parallel and by power users who want
-///      their own layout.
-///   2. `$XDG_RUNTIME_DIR/clide-<user>.sock` — Linux default; the
-///      per-user tmpfs lives exactly for this kind of short-lived
-///      socket and is auto-cleaned on logout.
-///   3. `/tmp/clide-<user>.sock` — fallback for environments without
-///      `XDG_RUNTIME_DIR`.
-String defaultSocketPath() {
-  final override = Platform.environment['CLIDE_SOCKET_PATH'];
-  if (override != null && override.isNotEmpty) return override;
+///   Linux:  `$XDG_RUNTIME_DIR/clide/<hash>.sock`
+///   macOS:  `$HOME/Library/Caches/clide/<hash>.sock`
+///
+/// The C `clide` client and any other consumer derive the same path
+/// from the same workspace root, so server + client always agree
+/// without configuration.
+String workspaceSocketPath(String workspaceRoot) {
+  final dir = socketDirectory();
+  return '$dir/${_hash(workspaceRoot)}.sock';
+}
+
+/// Parent directory that holds every per-workspace socket for this
+/// user. Created with `0700` on bind (see D-71). Exposed separately
+/// so the server can prepare/perm-fix the directory before binding.
+String socketDirectory() {
+  if (Platform.isMacOS) {
+    final home = Platform.environment['HOME'] ?? '/tmp';
+    return '$home/Library/Caches/clide';
+  }
   final xdg = Platform.environment['XDG_RUNTIME_DIR'];
-  final user = Platform.environment['USER'] ?? 'anon';
   final base = (xdg != null && xdg.isNotEmpty) ? xdg : '/tmp';
-  return '$base/clide-$user.sock';
+  return '$base/clide';
+}
+
+/// FNV-1a 64-bit, lower-case hex, fixed 16 chars. Matches the shape
+/// used by `lib/builtin/claude/src/session_naming.dart#_hash`. Not a
+/// cryptographic hash — D-70 explains why one isn't needed here.
+String _hash(String s) {
+  // 0xcbf29ce484222325 as two 32-bit halves to dodge JS-precision
+  // issues if this file ever runs under the web target.
+  var hiHi = 0xcbf2, hiLo = 0x9ce4;
+  var loHi = 0x8422, loLo = 0x2325;
+  const primeHiHi = 0x0000, primeHiLo = 0x0100;
+  const primeLoHi = 0x0000, primeLoLo = 0x01b3;
+  for (var i = 0; i < s.length; i++) {
+    loLo ^= s.codeUnitAt(i) & 0xffff;
+    // 64-bit multiply, hand-rolled across four 16-bit limbs.
+    final r0 = loLo * primeLoLo;
+    final r1 = (loLo * primeLoHi) + (loHi * primeLoLo) + (r0 >> 16);
+    final r2 = (loLo * primeHiLo) + (loHi * primeLoHi) + (hiLo * primeLoLo) + (r1 >> 16);
+    final r3 = (loLo * primeHiHi) + (loHi * primeHiLo) + (hiLo * primeLoHi) + (hiHi * primeLoLo) + (r2 >> 16);
+    loLo = r0 & 0xffff;
+    loHi = r1 & 0xffff;
+    hiLo = r2 & 0xffff;
+    hiHi = r3 & 0xffff;
+  }
+  String hex4(int v) => v.toRadixString(16).padLeft(4, '0');
+  return '${hex4(hiHi)}${hex4(hiLo)}${hex4(loHi)}${hex4(loLo)}';
 }