Optimize PackageLister version constraints with discrete bitmasks

[sigurdm]

Description

This PR optimizes version constraint evaluations in the version solver (PackageLister) by introducing discrete bitmasks (VersionBitmask).

Problem

Previously, PackageLister.countVersions and PackageLister.bestVersion performed linear traversals (.where((id) => constraint.allows(id.version)).length and reverse iteration loops) over the full version list of each candidate package. For mature packages with hundreds of versions (e.g. analyzer, http, intl, build_runner), this caused solving time and backtracking to scale with package age/version count ($O(N)$).

Solution

1. VersionBitmask (lib/src/solver/version_bitmask.dart):
   • Represents discrete version constraints over a sorted version list of length $N$ using 32-bit words (Uint32List).
   • Supports $O(1)$ bitwise operations: &, |, difference, allowsAll, allowsAny, count() (popcount), highestIndex(), lowestIndex(), and allows(index).
2. PackageLister (lib/src/solver/package_lister.dart):
   • Indexes package versions once into _PackageVersionIndex.
   • Constructs bitmasks for VersionConstraint instances via fast binary search (Version, VersionRange, VersionUnion, with SemVer pre-release normalization) and caches mask lookups.
   • Replaces linear traversals in countVersions() and bestVersion() with $O(1)$ bitmask queries.
3. Tests (test/solver/version_bitmask_test.dart):
   • 75 unit test cases covering bitmask operations across lengths [0, 1, 5, 62, 63, 64, 65, 128, 256, 1000], range boundaries, set operations, and equality.

Benchmarks (50,000 queries per config)

Package Versions ($N$)	Metric	Baseline (Linear)	Bitmask ($O(1)$)	Speedup
10 versions	countVersions	136.1 ms	43.4 ms	3.1x faster
	bestVersion	69.4 ms	42.3 ms	1.6x faster
	Combined	205.5 ms	85.7 ms	2.4x faster
30 versions	countVersions	436.6 ms	42.0 ms	10.4x faster
	bestVersion	75.8 ms	41.8 ms	1.8x faster
	Combined	512.5 ms	83.9 ms	6.1x faster
60 versions	countVersions	870.5 ms	43.0 ms	20.2x faster
	bestVersion	341.5 ms	42.7 ms	8.0x faster
	Combined	1,212.1 ms	85.7 ms	14.1x faster
150 versions	countVersions	2,074.5 ms	41.6 ms	49.9x faster
	bestVersion	1,228.3 ms	42.7 ms	28.8x faster
	Combined	3,302.8 ms	84.2 ms	39.2x faster
300 versions	countVersions	3,996.6 ms	46.7 ms	85.7x faster
	bestVersion	2,815.5 ms	47.2 ms	59.7x faster
	Combined	6,812.1 ms	93.8 ms	72.6x faster