Build Darling's system root store from the host CA bundle

Security reads the system trust domain from
/System/Library/Keychains/SystemRootCertificates.keychain and
SystemTrustSettings.plist (TrustSettingsSchema.h). Darling ships neither, so
SecTrustSettingsCopyCertificates(kSecTrustSettingsDomainSystem) returns
errSecNoTrustSettings (-25263); Chromium's known_roots_mac and Riot's
CertBundle then report that they cannot read the system roots.

ensure-darling-root-certificates.py parses each host root with a minimal DER
walker and writes the trust record keyed by the uppercase SHA-1, with the
encoded issuer Name, the serial INTEGER's content octets, modDate and an
empty (unrestricted) trustSettings array. The keychain is built with the
guest security tool in one darling shell, in a private temporary directory,
and published from inside the guest so a running Darling sees it, falling
back to the host when the guest cannot write the directory. A digest stamp
skips later runs without starting Darling. The launcher warns and continues
on failure; install-runtime.sh runs it before stopping Darling.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/TESTING.md

@@ -12,6 +12,7 @@ python3 tools/riot-launcher-offline-check.py
 python3 tools/riot-memory-installer-offline-check.py
 python3 tools/riot-foundation-installer-offline-check.py
 python3 tools/riot-llvm-version-offline-check.py
+python3 -m unittest tests.test_root_certificates
 ```
 
 The LLVM version check compiles Iridium's actual dynamic loader against small stub libraries. It verifies that a matching major initializes, older/newer majors produce a version mismatch, and a missing library fails initialization. It does not start Darling. The native AIR translator (`python3 tools/build-riot-air-check.py --native`) selects both headers and the shared library from the same `llvm-config`; its build prints those paths so shader results can be attributed to the tested LLVM installation.