modules: add dirtydecrypt (CVE-2026-31635) + fragnesia (CVE-2026-46300)

Two new page-cache-write LPE modules, both ported from the public V12
security PoCs (github.com/v12-security/pocs):

- dirtydecrypt (CVE-2026-31635): rxgk missing-COW in-place decrypt.
  rxgk_decrypt_skb() decrypts spliced page-cache pages before the HMAC
  check, corrupting the page cache of a read-only file. Sibling of
  Copy Fail / Dirty Frag in the rxrpc subsystem.

- fragnesia (CVE-2026-46300): XFRM ESP-in-TCP skb_try_coalesce() loses
  the SHARED_FRAG marker, so the ESP-in-TCP receive path decrypts
  page-cache pages in place. A latent bug exposed by the Dirty Frag
  fix (f4c50a4034e6). Retires the old _stubs/fragnesia_TBD stub.

Both wrap the PoC exploit primitive in the skeletonkey_module
interface: detect/exploit/cleanup, an --active /tmp sentinel probe,
--no-shell support, and embedded auditd + sigma rules. The exploit
body runs in a forked child so the PoC's exit()/die() paths cannot
tear down the dispatcher. The fragnesia port drops the upstream PoC's
ANSI TUI (incompatible with a shared dispatcher); the exploit
mechanism is reproduced faithfully. Linux-only code is guarded with
#ifdef __linux__ so the modules still compile on non-Linux dev boxes.

VERIFICATION: ported, NOT yet validated end-to-end on a
vulnerable-kernel VM. The CVE fix commits are not pinned, so detect()
is precondition-only (PRECOND_FAIL / TEST_ERROR, never a blind
VULNERABLE) and --auto will not fire them unless --active confirms.
macOS stub-path compiles verified locally; the Linux exploit-path
build is covered by CI (build.yml, ubuntu) only. See each MODULE.md.

Wiring: core/registry.h, skeletonkey.c, Makefile, CVES.md, ROADMAP.md.

modules/dirtydecrypt_cve_2026_31635/detect/sigma.yml

@@ -0,0 +1,29 @@
+title: Possible DirtyDecrypt exploitation (CVE-2026-31635)
+id: 7c1e9a40-skeletonkey-dirtydecrypt
+status: experimental
+description: |
+  Detects the file-modification footprint of the rxgk page-cache write
+  (DirtyDecrypt / DirtyCBC, CVE-2026-31635): non-root creation of
+  AF_RXRPC sockets followed by modification of a setuid-root binary or
+  a credential file.
+references:
+  - https://github.com/v12-security/pocs/tree/main/dirtydecrypt
+logsource:
+  product: linux
+  service: auditd
+detection:
+  modification:
+    type: 'PATH'
+    name|startswith:
+      - '/usr/bin/su'
+      - '/bin/su'
+      - '/etc/passwd'
+      - '/etc/shadow'
+  not_root:
+    auid|expression: '!= 0'
+  condition: modification and not_root
+level: high
+tags:
+  - attack.privilege_escalation
+  - attack.t1068
+  - cve.2026.31635