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/NOTICE.md

@@ -0,0 +1,45 @@
+# NOTICE — dirtydecrypt
+
+## Vulnerability
+
+**CVE-2026-31635** — "DirtyDecrypt" / "DirtyCBC". Missing copy-on-write
+guard in `rxgk_decrypt_skb()` (`net/rxrpc/rxgk_common.h`). The function
+calls `skb_to_sgvec()` then `crypto_krb5_decrypt()` with no
+`skb_cow_data()`; the `krb5enc` AEAD template (`crypto/krb5enc.c`)
+decrypts **in place** before verifying the HMAC. When the skb fragment
+pages are page-cache pages (spliced in via `MSG_SPLICE_PAGES` over
+loopback), the in-place decrypt corrupts the page cache of a read-only
+file. The same pattern exists in rxkad (`rxkad_verify_packet_2`).
+
+Sibling of Copy Fail (CVE-2026-31431) and Dirty Frag
+(CVE-2026-43284 / CVE-2026-43500) — all are page-cache write
+primitives that abuse a missing COW boundary.
+
+## Research credit
+
+Discovered and reported by the **Zellic** and **V12 security** team.
+Public proof-of-concept by **Luna Tong** ("cts" / "gf_256"), Zellic
+co-founder, on the V12 team.
+
+> Reference PoC: <https://github.com/v12-security/pocs/tree/main/dirtydecrypt>
+
+On disclosure (2026-05-09) the kernel maintainers indicated the issue
+duplicated a flaw already patched in mainline; CVE-2026-31635 was
+assigned subsequently.
+
+## SKELETONKEY role
+
+`skeletonkey_modules.c` is a port of the V12 PoC into the
+`skeletonkey_module` interface. The exploit primitive — the
+`fire()` / `pagecache_write()` sliding-window machinery, the rxgk XDR
+token builder, the 120-byte ET_DYN ELF payload — is reproduced from
+that PoC. SKELETONKEY adds the detect/cleanup lifecycle, an `--active`
+sentinel probe, `--no-shell` support, and the embedded detection
+rules. Research credit belongs to the people above.
+
+## Verification status
+
+**Ported, not yet validated end-to-end on a vulnerable-kernel VM.**
+The CVE-2026-31635 fix commit is not yet pinned in this module, so
+`detect()` does not perform a kernel-version patched/vulnerable
+verdict — see `MODULE.md`.