schfs: records at arbitrary offsets are packed, and the compiler is told

A leaf's value area holds variable-length records end to end, so a record begins
wherever the one before it finished -- a multiple of 8 only by luck. The code
reads them by casting a pointer into the block, which tells the compiler they
are aligned when they are not.

That works on x86, which is why it has never shown as a bug, and it is undefined
in C. A compiler entitled to assume 8-byte alignment may emit an instruction
that requires it, and the failure would arrive at a different optimisation level
or on a different architecture looking like filesystem corruption rather than
like codegen. UndefinedBehaviorSanitizer reported 31 of them across obj.c,
objw.c, attr.c and space.c.

Seven types now carry SCHFS_PACKED: dirent_rec, extent, extref, index_rec,
attr_rec, snap and inode. Two of those -- attr_rec and inode -- were never
reported, because schfs_objtest does not reach an unaligned instance of either.
They were latent rather than absent, and casting into a leaf is the same lie
whether or not a test has caught it yet.

NO ON-DISK LAYOUT CHANGED, and that was measured before the annotation was
applied rather than assumed: every one of these already carries explicit `pad`
fields and therefore has no implicit padding, so sizes and member offsets are
identical with and without it. Seven _Static_asserts now pin the sizes, because
a record that silently grows misreads every existing volume from that point on
and looks like corruption rather than like a struct that changed.

schfs_node_hdr, schfs_item, schfs_kptr and schfs_uberblock are deliberately left
alone. The node header is exactly 32 bytes, so the items and key pointers after
it are already 8-aligned, and annotating them would cost byte-wise access on the
tree's hottest paths to fix a problem they do not have.

Verified by reading an image written before the change: schfs_fsck --check
passes on structure, checksums, key ordering, data blocks and derived free
space, and the kernel mounts its own root unchanged.

Also here: mkschfs passed a null pointer to qsort for an empty directory. Every
implementation ignores it when the count is zero; the standard does not say they
have to.

make schfs-check now runs with no sanitizer output at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

tools/mkschfs.c

@@ -584,7 +584,11 @@ add_tree(uint64_t dirino, const char *hostdir)
     }
     closedir(d);
 
-    qsort(l.v, l.n, sizeof(*l.v), nl_cmp);
+    // qsort declares its first argument non-null, and an empty directory
+    // leaves l.v at NULL. Every implementation ignores the pointer when the
+    // count is zero; the standard does not say they have to.
+    if (l.n > 0)
+        qsort(l.v, l.n, sizeof(*l.v), nl_cmp);
     long subdirs = 0;
     for (int i = 0; i < l.n; i++) {
         subdirs += add_entry(dirino, hostdir, l.v[i]);