Fix real memory leak Gitea CI caught, that local testing had been masking

Gitea CI flagged two things on the last push:

1. A -Wunused-result warning on an intentionally-ignored write() return
   value in test_crash_consistency.c's progress side-channel. Fixed with
   an explicit (void) cast and a comment explaining why ignoring it is
   safe (a short write there only makes the progress count more
   conservative, per that function's own existing documented tolerance).

2. A real LeakSanitizer failure -- 256 bytes across 4 allocations from
   vfs_new/vfs_unmount. Root cause: tests/test_journal_failure.c's two
   "reopen after the failure, verify recovery" blocks called
   vfs_unmount/backend_free/backend_free inside their `if (ov2)` branch
   (the normal, expected path) but vfs_free(v2) only on the `else`
   branch, which is never actually reached in practice. Fixed by moving
   vfs_free(v2) to run unconditionally after the if, in both blocks.

This bug was invisible locally across many runs because local sanitizer
verification had been using ASAN_OPTIONS=detect_leaks=0 -- adopted
originally for a real reason (a SIGKILLed forked child in
test_crash_consistency.c never runs its own exit-time leak check, so its
allocations were never the actual concern) but applied to the whole test
run, which also suppressed detection of this real bug in the *parent*
process's own code. CI doesn't set that option, so it caught what local
runs couldn't. Documented in CONTRIBUTING.md as a real process gap, not
just a code bug: a local verification habit that diverges from what CI
actually runs can let a real finding through until it reaches CI.

Verified: confirmed the leak directly first (reproduced locally by
dropping the detect_leaks=0 override, matching CI exactly, before
touching any code), then confirmed the fix by re-running the same
no-override sweep across all 8 test binaries with zero leaks found, plus
a clean make all + make test.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB

tests/test_crash_consistency.c

@@ -221,7 +221,11 @@ static void record_progress(const char *progress_path, int n_done) {
     if (fd < 0) return;
     char buf[16];
     int len = snprintf(buf, sizeof(buf), "%d", n_done);
-    write(fd, buf, (size_t)len);
+    /* Best-effort: a short write here only makes read_progress()'s count
+     * more conservative (see the imprecision note above), never wrong in
+     * the unsafe direction, so there is nothing more useful to do with a
+     * partial-write return value than the explicit (void) already says. */
+    (void)write(fd, buf, (size_t)len);
     fsync(fd);
     close(fd);
 }

tests/test_journal_failure.c

@@ -124,9 +124,8 @@ int main(void) {
             vfs_unmount(v2, "/");
             backend_free(ov2);
             backend_free(mem2);
-        } else {
-            vfs_free(v2);
         }
+        vfs_free(v2);
     }
 
     /* --- scenario 2: vfs_mkdir's journal append fails --- */
@@ -198,9 +197,8 @@ int main(void) {
             vfs_unmount(v2, "/");
             backend_free(ov2);
             backend_free(mem2);
-        } else {
-            vfs_free(v2);
         }
+        vfs_free(v2);
     }
 
     reset_paths(pack_path, jpath);