verify-vm: kernel.ubuntu.com mainline integration — 22 modules verified

Unblocks the 4 previously-PIN_FAIL modules by adding a fallback path to
kernel.ubuntu.com/mainline/ for any kernel no longer in apt. Adds 4 more
matches to the verified_on table for a total of 22 modules confirmed
against real Linux VMs:

  af_unix_gc     ubuntu2204 + mainline 5.15.5  match
  nf_tables      ubuntu2204 + mainline 5.15.5  match
  nft_set_uaf    ubuntu2204 + mainline 5.15.5  match
  stackrot       ubuntu2204 + mainline 6.1.10  match

Mechanism:

  tools/verify-vm/Vagrantfile — new 'pin-mainline-<X.Y.Z>' shell
  provisioner. Fetches the directory index at
  https://kernel.ubuntu.com/mainline/v<X.Y.Z>/amd64/, parses out the 4
  canonical .deb filenames (linux-headers _all, linux-headers
  -generic _amd64, linux-image-unsigned -generic _amd64, linux-modules
  -generic _amd64; skips lowlatency), downloads them, runs 'dpkg -i' +
  'update-grub', and prints a reboot hint.

  Mainline package version like '5.15.5-051505' sorts ABOVE Ubuntu's
  stock '5.15.0-91' in debian-version-compare (numeric 51505 > 91), so
  update-grub puts it at the top of the boot menu and the next
  'vagrant reload' lands on it automatically. uname then reports
  '5.15.5-051505-generic' which our parser sees as 5.15.5 → in our
  kernel_range table's vulnerable window → empirical VULNERABLE.

  tools/verify-vm/verify.sh — new SKK_VM_MAINLINE_VERSION env passed to
  the Vagrantfile. Reload trigger now also fires when uname doesn't
  match the mainline target.

  tools/verify-vm/targets.yaml — new 'mainline_version' field on the 4
  PIN_FAIL targets. kernel_pkg is left empty; mainline_version drives
  the fetch. Picked 5.15.5 (Nov 2021) for the 5.15-line CVEs and
  6.1.10 (Feb 2023) for stackrot — both below every relevant backport.

Final sweep status (22 of 26 CVEs):

  ✓ MATCHES (22):
    pwnkit, cgroup_release_agent, netfilter_xtcompat, fuse_legacy,
    nft_fwd_dup, entrybleed, overlayfs, overlayfs_setuid,
    sudoedit_editor, ptrace_traceme, sudo_samedit, af_packet,
    pack2theroot, cls_route4, nft_payload, af_packet2, sequoia,
    dirty_pipe, nf_tables, af_unix_gc, nft_set_uaf, stackrot

  🚫 NOT VERIFIED (4 — flagged in targets.yaml with rationale):
    vmwgfx        — VMware-guest only; no public Vagrant box covers it
    dirtydecrypt  — needs Linux 7.0; not shipping as any distro kernel
    fragnesia     — needs Linux 7.0; same
    dirty_cow     — needs ≤ 4.4 kernel; older than every supported
                    Vagrant box (would need a custom image)

  copy_fail_family entries verified indirectly via the shared
  infrastructure tests in the kernel_range unit-test harness.

The 22 records are baked into core/verifications.c and surface in
--list (VFY ✓ column), --module-info (--- verified on --- section),
--explain (VERIFIED ON section), and JSON output (verified_on array).
22/26 CVEs is the new trust signal; with the mainline fetch path
production-ready, additional pin targets can be added to targets.yaml
without code changes.

core/verifications.c

@@ -36,6 +36,16 @@ const struct verification_record verifications[] = {
         .actual_detect = "VULNERABLE",
         .status        = "match",
     },
+    {
+        .module        = "af_unix_gc",
+        .verified_at   = "2026-05-23",
+        .host_kernel   = "5.15.5-051505-generic",
+        .host_distro   = "Ubuntu 22.04.3 LTS",
+        .vm_box        = "generic/ubuntu2204",
+        .expect_detect = "VULNERABLE",
+        .actual_detect = "VULNERABLE",
+        .status        = "match",
+    },
     {
         .module        = "cgroup_release_agent",
         .verified_at   = "2026-05-23",
@@ -96,6 +106,16 @@ const struct verification_record verifications[] = {
         .actual_detect = "VULNERABLE",
         .status        = "match",
     },
+    {
+        .module        = "nf_tables",
+        .verified_at   = "2026-05-23",
+        .host_kernel   = "5.15.5-051505-generic",
+        .host_distro   = "Ubuntu 22.04.3 LTS",
+        .vm_box        = "generic/ubuntu2204",
+        .expect_detect = "VULNERABLE",
+        .actual_detect = "VULNERABLE",
+        .status        = "match",
+    },
     {
         .module        = "nft_fwd_dup",
         .verified_at   = "2026-05-23",
@@ -116,6 +136,16 @@ const struct verification_record verifications[] = {
         .actual_detect = "VULNERABLE",
         .status        = "match",
     },
+    {
+        .module        = "nft_set_uaf",
+        .verified_at   = "2026-05-23",
+        .host_kernel   = "5.15.5-051505-generic",
+        .host_distro   = "Ubuntu 22.04.3 LTS",
+        .vm_box        = "generic/ubuntu2204",
+        .expect_detect = "VULNERABLE",
+        .actual_detect = "VULNERABLE",
+        .status        = "match",
+    },
     {
         .module        = "overlayfs",
         .verified_at   = "2026-05-23",
@@ -176,6 +206,16 @@ const struct verification_record verifications[] = {
         .actual_detect = "VULNERABLE",
         .status        = "match",
     },
+    {
+        .module        = "stackrot",
+        .verified_at   = "2026-05-23",
+        .host_kernel   = "6.1.10-060110-generic",
+        .host_distro   = "Ubuntu 22.04.3 LTS",
+        .vm_box        = "generic/ubuntu2204",
+        .expect_detect = "VULNERABLE",
+        .actual_detect = "VULNERABLE",
+        .status        = "match",
+    },
     {
         .module        = "sudo_samedit",
         .verified_at   = "2026-05-23",