v0.3.1: --dump-offsets tool + NOTICE.md per module

The README has been claiming "each module credits the original CVE
reporter and PoC author in its NOTICE.md" since v0.1.0, but only
copy_fail_family actually shipped one. Fixed.

  modules/<name>/NOTICE.md (×19 new + 1 existing): per-module
    research credit covering CVE ID, discoverer, original advisory
    URL where public, upstream fix commit, IAMROOT's role.

  iamroot.c: new --dump-offsets subcommand. Resolves kernel offsets
    via the existing core/offsets.c four-source chain (env →
    /proc/kallsyms → /boot/System.map → embedded table), then emits
    a ready-to-paste C struct entry for kernel_table[]. Run once
    as root on a target kernel build; upstream via PR. Eliminates
    fabricating offsets — every shipped entry traces back to a
    `iamroot --dump-offsets` invocation on a real kernel.

  docs/OFFSETS.md: documents the --dump-offsets workflow.
  CVES.md: notes the NOTICE.md convention + offset dump tool.

  iamroot.c: bump IAMROOT_VERSION 0.3.0 → 0.3.1.

modules/overlayfs_cve_2021_3493/NOTICE.md

@@ -0,0 +1,25 @@
+# NOTICE — overlayfs (CVE-2021-3493)
+
+## Vulnerability
+
+**CVE-2021-3493** — Ubuntu overlayfs userns file-capability injection
+→ host root via setcap'd binaries in a userns-mounted overlay.
+
+## Research credit
+
+Reported by **Vasily Kulikov**, April 2021. Ubuntu-specific because
+upstream didn't enable unprivileged userns-overlayfs-mount until 5.11.
+
+Advisory: USN-4915-1 / USN-4916-1 (Canonical, April 2021).
+
+Public PoC: vsh-style userns + overlayfs + xattr injection chain.
+
+## IAMROOT role
+
+Detect parses `/etc/os-release` for `ID=ubuntu`, checks
+`unprivileged_userns_clone` sysctl, and with `--active` performs the
+mount as a fork-isolated probe. The full exploit performs the
+userns+overlayfs mount, plants a setcap'd carrier binary in the
+upper layer, and execs it from the unprivileged side to obtain root
+on the host. Ships auditd rules covering `mount(overlay)` and
+`setxattr(security.capability)`.