Add canonical frontend FSM runner

[deevus]

Summary

• add a flat canonical FSM runner and optional overlay stack adapter
• refactor the top-level frontend lifecycle to use the runner while preserving public globals/API
• add focused FSM/stack tests and wire new sources into Zig, CMake, and roller-core manifests

Verification

• git diff --check
• zig build test-fsm
• zig build
• python3 -m unittest tests.test_cmake_roller_core
• python3 tools/check_roller_core_manifest.py
• python3 tools/check_source_set_drift.py

Known unrelated failures

• zig build test still fails because docs/adr/0003 is missing
• zig build test still fails in test-f-s3-soak when FATDATA/TRACK3.TRK is unavailable

[github-actions]

ROLLER browser preview: https://worktree-canonical-state-mac.roller-web.pages.dev

Updated for commit a118d6a.