Encapsulate frontend state in FSM API

[deevus]

Stack

• Depends on Add canonical frontend FSM runner #288

Summary

• expose FSM-backed frontend current, pending, and request functions
• migrate frontend callers away from direct state globals
• remove the legacy state globals and synchronization bridge
• add a source contract for the public API and legacy-symbol removal

Verification

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

Known unrelated full-suite failures

• missing docs/adr/0003-canonical-geometry-conventions.md
• missing FATDATA/TRACK3.TRK for editor_track_loader_test

Review

Independent semantic review found no blockers. Residual risk: frontend wrapper behavior is covered by lower-level FSM tests and compilation, while the new frontend API test is a source contract rather than a linked integration test.

[github-actions]

ROLLER browser preview: https://worktree-remove-frontend-leg.roller-web.pages.dev

Updated for commit 2c0e5d7.