test(core): cover artifact flow and visual safety

[yashranaway]

Progresses #36

Summary

• cover exact-bound artifact reads plus rejection of oversized, wrong-extension, symlink, and directory inputs
• lock in the security property that fill and other sensitive/destructive commands never become replayable flow steps
• exercise visual comparison through a controlled executable, including its one-frame difference arguments and failure path
• update the D2 progress record without closing the remaining umbrella gaps

Verification

• CI-equivalent Swift 6.1 container: 36/36 protocol tests plus MCP integration passed
• npx --yes pnpm@9.15.0 test:runtime
• git diff --check