docs(nix): close Quint duplication optimization and record next bottlenecks

[davidabram]

Summary

• Mark redundant Quint MBT compilation/execution as already resolved by perf(nix): speed up Rust checks and deduplicate Quint MBT #308 and ci(quint): run spec verification without recompiling Rust MBT #309; remove it from the outstanding optimization backlog.
• Correct the stale documentation saying the Quint workflow invokes the focused Rust MBT runner: it now runs Quint spec verification only, while cli-tests runs MBT once.
• Record preliminary profiling evidence (270.3 s wall, 269.8 s critical path; approx. 115 s test binary compilation) and prioritize measurable follow-ups.

Scope

Documentation only (context/sce/flake-build-performance.md). No runtime, Nix check, or CI workflow changes. The local uncommitted profiler files are not included.

Validation

• Verified the parent ci(nix): record flake, build, and smoke timings in job summaries #311 stack's flake.nix and .github/workflows/quint.yml reflect the perf(nix): speed up Rust checks and deduplicate Quint MBT #308/ci(quint): run spec verification without recompiling Rust MBT #309 deduplication.
• Cross-checked all documented figures against the supplied source-change profile. The estimates are clearly marked preliminary and not direct per-crate timings.
• Documentation-only; no Nix builds were run for this PR.

Stacked on #311 (perf/ci-stack-3-timing-diagnostics).