perf(database): index migrations by name

[xHeaven]

rehashAll() and validate() were rebuilding the full migrations list for every migration row in the database. So with 500 migrations, it copied/scanned the 500-item list 500 times instead of building it once.

[github-actions]

Benchmark Results

Comparison of perf/migration-lookup against 3.x (e667dcc305364b47b2588c50eaaaaa29462ec896).

Open to see the benchmark results

Benchmark	Set	Mem. Peak	Time	Variability
ViewRenderBench(benchExpressions)	-	24.532mb 0.00%	487.034μs -6.39%	±3.13% +89.69%
ViewRenderBench(benchControlFlow)	-	44.442mb 0.00%	665.058μs -5.63%	±3.27% +31.17%

_{Generated by phpbench against commit 2c3b83e}

[brendt]

Feel free to merge when CI is done :)