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/fragnesia_cve_2026_46300/NOTICE.md

@@ -0,0 +1,48 @@
+# NOTICE — fragnesia
+
+## Vulnerability
+
+**CVE-2026-46300** — "Fragnesia" ("Fragment Amnesia"). XFRM ESP-in-TCP
+local privilege escalation. `skb_try_coalesce()` fails to propagate the
+`SKBFL_SHARED_FRAG` marker when moving paged fragments between socket
+buffers, so the kernel loses track of the fact that a fragment is
+externally backed by page-cache pages spliced in from a file. The
+ESP-in-TCP receive path then decrypts in place, corrupting the page
+cache of a read-only file.
+
+Fragnesia is a **latent bug exposed by the Dirty Frag remediation**:
+the candidate fix explicitly cites the Dirty Frag patch
+(`f4c50a4034e6`) as a commit it "fixes" — the Dirty Frag remediation
+made a previously latent flaw practically exploitable.
+
+## Research credit
+
+Discovered by **William Bowling** with the **V12 security** team.
+
+> Reference PoC: <https://github.com/v12-security/pocs/tree/main/fragnesia>
+> Patch thread: <https://lists.openwall.net/netdev/2026/05/13/79>
+
+## SKELETONKEY role
+
+`skeletonkey_modules.c` is a port of the V12 PoC
+(`xfrm_espintcp_pagecache_replace`) into the `skeletonkey_module`
+interface. The exploit primitive — the AES-GCM keystream-byte table
+built via AF_ALG, the per-byte IV selection, the userns + netns + XFRM
+ESP-in-TCP setup, the splice-driven sender/receiver trigger pair, the
+192-byte ELF payload — is reproduced from that PoC.
+
+**Port adaptation:** the PoC's ANSI "smash frame" TUI
+(`draw_smash_frame` + terminal scroll-region escape sequences) is
+**not** carried over — it is incompatible with running as one module
+among many under a shared dispatcher. Progress is reported with
+SKELETONKEY's `[*]`/`[+]`/`[-]` log prefixes instead. SKELETONKEY also
+adds the detect/cleanup lifecycle, an `--active` 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.**
+Requires `CONFIG_INET_ESPINTCP` and unprivileged user-namespace
+creation. The CVE-2026-46300 fix commit is not yet pinned in this
+module — see `MODULE.md`.