fix: avoid redundant Nautilus completion pass - #1652

Summary

Nautilus searches that converged with new posterior samples previously performed an unnecessary intermediate result update and a second no-op sampler call. Use Nautilus's convergence return value to finish immediately, and advance finite update budgets using the cumulative likelihood count instead of posterior sample counts. Global limits also stop when a batch overshoots them.

Closes #1651.

API Changes

No argument or signature changes. Converged searches skip the redundant intermediate update; finite update cadence and global limits count likelihood evaluations correctly. No workspace migration is required.

Test Plan

• Targeted Nautilus tests: 16 passed, including 8 new convergence/budget cases.
• All 8 new regression cases fail against the original methods.
• Full serial PyAutoFit suite: 2934 passed, 2 skipped (300.23 s).
• Seeded real Nautilus fits (seeds 13 and 14): sampler calls 2 → 1 and intermediate updates 1 → 0; posterior parameters, likelihoods and weights bit-identical to the original wrapper.
• Timing caveat: these two small runs do not establish a wall-time speedup (5.47–6.04 s baseline, 5.68–6.15 s fixed, with concurrent suite activity).
• Instrumented repeat: the removed intermediate update took 68–75 ms per search; seeded samples remained bit-identical. Total timing differences are noisy, so no end-to-end speedup is claimed.
• Downstream smoke: 160 scripts and 6 notebooks passed, zero failures, across autofit, autogalaxy, autolens, autolens_test, euclid, howtolens, autocti and autocti_test. Each workspace used its isolated Heart environment and disposable checkout with this branch's PyAutoFit source.

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