G18: Linux-first startup performance design and delivery

[zhnt]

Requested outcome

Design and review Linux-first startup performance improvements on the delivered G17 baseline. Use a dedicated harness task branch and isolated test environment. Develop Linux optimizations before coordinated macOS/Windows validation on other machines.

Current scope

• Refresh the startup performance plan against main 9bc6936 and measured Linux facts.
• Define measurement, import/facade and CLI dispatch boundaries without changing Product readiness or lifecycle.
• Independently review architecture/parallel ownership, measurement validity, and compatibility/lifecycle risks; resolve design findings before implementation.
• Keep Product execution/ledger work in issue appservice: optional Product execution contract and deterministic lifecycle validation #576 separate; do not use performance work to hide its unresolved G10 comparison.

Boundaries

No implementation activation, main merge, push, default Hosted migration, timeout relaxation, live provider tests, first-frame/readiness reordering, background import, or deployment/bytecode policy change in this design step. Linux evidence is not cross-platform acceptance.