module metadata: CWE + ATT&CK + CISA KEV triage from federal sources

Adds per-CVE triage annotations that turn SKELETONKEY's JSON output
into something a SIEM/CTI/threat-intel pipeline can route on, and a
KEV badge in --list so operators see at-a-glance which modules
cover actively-exploited bugs.

New tool — tools/refresh-cve-metadata.py:

  - Discovers CVEs by scanning modules/<dir>/ (no hardcoded list).
  - Fetches CISA's Known Exploited Vulnerabilities catalog
    (https://www.cisa.gov/.../known_exploited_vulnerabilities.csv).
  - Fetches CWE classifications from NVD's CVE API 2.0
    (services.nvd.nist.gov), throttled to the anonymous
    5-req/30s limit (~3 minutes for 26 CVEs).
  - Hand-curated ATT&CK technique mapping (T1068 default; T1611 for
    container escapes, T1082 for kernel info leaks — MITRE doesn't
    publish a clean CVE→technique feed).
  - Generates three outputs:
      docs/CVE_METADATA.json   machine-readable, drift-checkable
      docs/KEV_CROSSREF.md     human-readable table
      core/cve_metadata.c      auto-generated lookup table
  - --check mode diffs the committed JSON against a fresh fetch for
    CI drift detection.

New core API — core/cve_metadata.{h,c}:

  struct cve_metadata { cve, cwe, attack_technique, attack_subtechnique,
                        in_kev, kev_date_added };
  const struct cve_metadata *cve_metadata_lookup(const char *cve);

Lookup keyed by CVE id, not module name — the metadata is properties
of the CVE (two modules covering the same bug see the same metadata).
The opsec_notes field stays on the module struct because exploit
technique varies per-module (different footprints).

Output surfacing:
  - --list: new KEV column shows ★ for KEV-listed CVEs.
  - --module-info (text): prints cwe / att&ck / 'in CISA KEV: YES (added
    YYYY-MM-DD)' between summary and operations.
  - --module-info / --scan (JSON): emits a 'triage' subobject with the
    full record, plus an 'opsec_notes' field at top level when set.

Initial snapshot:
  - 10 of 26 modules cover KEV-listed CVEs (dirty_cow, dirty_pipe,
    pwnkit, sudo_samedit, ptrace_traceme, fuse_legacy, nf_tables,
    overlayfs, overlayfs_setuid, netfilter_xtcompat).
  - 24 of 26 have NVD CWE mappings; 2 unmapped (NVD has no weakness
    record for CVE-2019-13272 and CVE-2026-46300 yet).
  - All 26 mapped to an ATT&CK technique.

Verification:
  - macOS local: 33 kernel_range + clean build, --module-info shows
    'in CISA KEV: YES (added 2024-05-30)' for nf_tables, --list KEV
    column renders.
  - Linux (docker gcc:latest): 33 + 54 = 87 passes, 0 fails.

Follow-up commits will add per-module OPSEC notes and --explain mode.

Makefile

@@ -20,7 +20,8 @@ BUILD   := build
 BIN     := skeletonkey
 
 # core/
-CORE_SRCS := core/registry.c core/kernel_range.c core/offsets.c core/finisher.c core/host.c
+CORE_SRCS := core/registry.c core/kernel_range.c core/offsets.c core/finisher.c \
+             core/host.c core/cve_metadata.c
 CORE_OBJS := $(patsubst %.c,$(BUILD)/%.o,$(CORE_SRCS))
 
 # Register-every-module helper. Lives in its own translation unit so