fix(errors): create cache dir before writing lock file

.omo/evidence/task-26-tofu-core.log

@@ -152,3 +152,32 @@ install failed: conflict detectedtestpkg-2.1 is already installed -- use -ReProv
   Up-to-date toml 1.0.0: target for configuration [library] is up to date.
   Up-to-date tofu ~main: target for configuration [application] is up to date.
     Finished To force a rebuild of up-to-date targets, run again with --force
+ ok  build plan: 1 packages
+==> generating build plan
+  -    + libfoo (recipe)
+  -    + mypkg (recipe)
+ ok  build plan: 2 packages
+23 modules passed unittests
+=== Re-verification after crash fix ===
+--- rm -rf ~/.cache/tofu && ./tofu --help ---
+tofu — ZUUR package manager
+Usage: tofu <command> [arg] [flags]
+Commands:
+  -S <pkg>     Install package from ZUUR recipes
+  -Ss <q>      Search ZUUR
+  -Syu         Upgrade installed packages
+  -R <pkg>     Remove package
+  -Si <pkg>    Show package info
+  -h, --help   Show this help
+Flags:
+  --noconfirm  Skip confirmation prompts
+  --dry-run    Show what would happen without doing it
+  --force      Overwrite existing builds
+  -j<N>        Parallel build jobs
+
+EXIT: 0
+--- ./tofu (no args) ---
+error no command given (run 'tofu --help')
+EXIT: 0
+--- dub test result ---
+23 modules passed unittests

src/tofu/errors.d

@@ -22,7 +22,7 @@ import tofu.config : ConfigException;
 import tofu.cli : CliException;
 
 import std.process : thisProcessID;
-import std.file : exists, readText, write, remove;
+import std.file : exists, readText, write, remove, mkdirRecurse;
 import std.path : buildPath;
 import std.conv : to, ConvException;
 import std.stdio : stderr;
@@ -187,6 +187,22 @@ private @trusted void removeLockFile(string path)
     catch (Exception) {}
 }
 
+/// Ensure the cache directory exists, creating it if necessary.
+/// Idempotent — `mkdirRecurse` succeeds if the directory already exists.
+private @trusted void ensureCacheDir(string cacheDir)
+{
+    try
+    {
+        mkdirRecurse(cacheDir);
+    }
+    catch (Exception e)
+    {
+        // Permission denied, path isn't a directory, etc.
+        throw new LockException(
+            "cannot create cache directory " ~ cacheDir ~ ": " ~ e.msg);
+    }
+}
+
 // ────────────────────────────────────────────────────────────
 // PID-liveness check
 // ────────────────────────────────────────────────────────────
@@ -267,6 +283,10 @@ private @safe string lockPath(string cacheDir)
 {
     import tofu.log : logWarn;
 
+    // Ensure the cache directory exists before writing the lock file.
+    // On a fresh system ~/.cache/tofu may not exist yet.
+    ensureCacheDir(cacheDir);
+
     auto path = lockPath(cacheDir);
     auto myPid = thisProcessID.to!string;