core/host: shared host fingerprint refactor

Adds core/host.{h,c} — a single struct skeletonkey_host populated once
at startup and handed to every module callback via ctx->host. Replaces
the per-detect uname / /etc/os-release / sysctl / userns-fork-probe
calls scattered across the corpus with O(1) cached lookups, and gives
the dispatcher one consistent view of the host.

What's in the fingerprint:

- Identity: kernel_version (parsed from uname.release), arch (machine),
  nodename, distro_id / distro_version_id / distro_pretty (parsed once
  from /etc/os-release).
- Process state: euid, real_uid (defeats userns illusion via
  /proc/self/uid_map), egid, username, is_root, is_ssh_session.
- Platform family: is_linux, is_debian_family, is_rpm_family,
  is_arch_family, is_suse_family (file-existence checks once).
- Capability gates (Linux): unprivileged_userns_allowed (live
  fork+unshare probe), apparmor_restrict_userns,
  unprivileged_bpf_disabled, kpti_enabled, kernel_lockdown_active,
  selinux_enforcing, yama_ptrace_restricted.
- System services: has_systemd, has_dbus_system.

Wiring:

- core/module.h forward-declares struct skeletonkey_host and adds the
  pointer to skeletonkey_ctx. Modules opt-in by including
  ../../core/host.h.
- core/host.c is fully POD (no heap pointers) — uses a single file-
  static instance, returns a stable pointer on every call. Lazily
  populated on first skeletonkey_host_get().
- skeletonkey.c calls skeletonkey_host_get() at main() entry, stores
  in ctx.host before any register_*() runs.
- cmd_auto's bespoke distro-fingerprint code (was an inline
  read_os_release helper) is replaced with skeletonkey_host_print_banner(),
  which emits a two-line banner of identity + capability gates.

Migrations:

- dirtydecrypt: kernel_version_current() -> ctx->host->kernel.
- fragnesia: removed local fg_userns_allowed() fork-probe in favour of
  ctx->host->unprivileged_userns_allowed (no per-scan fork). Also
  pulls kernel from ctx->host. The PRECOND_FAIL message now notes
  whether AppArmor restriction is on.
- pack2theroot: access('/etc/debian_version') -> ctx->host->is_debian_family;
  also short-circuits when ctx->host->has_dbus_system is false (saves
  the GLib g_bus_get_sync attempt on systems without system D-Bus).
- overlayfs: replaced the inline is_ubuntu() /etc/os-release parser
  with ctx->host->distro_id comparison. Local helper preserved for
  symmetry / standalone builds.

Documentation: docs/ARCHITECTURE.md gains a 'Host fingerprint'
section describing the struct, the opt-in include pattern, and
example detect() usage. ROADMAP --auto accuracy log notes the
landing and flags remaining modules as an incremental follow-up.

Build verification:

- macOS (local): make clean && make -> Mach-O x86_64, 31 modules,
  banner prints with distro=?/? (no /etc/os-release).
- Linux (docker gcc:latest + libglib2.0-dev): make clean && make ->
  ELF 64-bit, 31 modules. Banner prints with kernel + distro=debian/13
  + 7 capability gates. dirtydecrypt correctly says 'predates the
  rxgk code added in 7.0'; fragnesia PRECOND_FAILs with
  '(host fingerprint)' annotation; pack2theroot PRECOND_FAILs on
  no-DBus; overlayfs reports 'not Ubuntu (distro=debian)'.

ROADMAP.md

@@ -227,6 +227,22 @@ of the 28-module verified corpus):**
       `a2567217` (Linux 7.0); `fragnesia` against 7.0.9; `pack2theroot`
       against PackageKit 1.3.5. The `kernel_range` model now drives
       their verdicts; `--active` confirms empirically on top.
+- [x] **`core/host` host-fingerprint refactor.** A single
+      `struct skeletonkey_host` is populated once at startup and
+      handed to every module via `ctx->host`: kernel version + arch
+      + distro id/version + capability gates (unprivileged_userns,
+      AppArmor restriction, BPF disabled, KPTI, lockdown, SELinux,
+      Yama ptrace) + service presence (systemd, system D-Bus). The
+      `--auto` / `--scan` banner now prints the fingerprint up front
+      so operators see at a glance which gates are open. 4 modules
+      migrated to consume the fingerprint (dirtydecrypt, fragnesia,
+      pack2theroot, overlayfs) — replacing per-detect `uname`s,
+      `/etc/os-release` parses, and userns fork-probes with O(1)
+      cached lookups. See `docs/ARCHITECTURE.md` for the pattern;
+      future modules can opt-in by including `core/host.h`.
+- [ ] Migrate the remaining modules (cgroup_release_agent /
+      overlayfs_setuid / copy_fail_family bridge / others) to
+      consume `ctx->host` — incremental follow-up.
 
 **Carry-overs:**