feat: add semver module for unified version handling

[Sunrisepeak]

Summary

• Add xlings::semver module with version parsing, comparison, and range matching
• Replace ad-hoc version handling in catalog, resolver, and xvm commands
• Fix lexicographic comparison bug (e.g. "9.0" > "15.0" was wrong)
• Support version range expressions: ^, ~, >=, >, <, <=, *

Supported syntax

1.2.3          exact match
15             prefix range [15.0.0, 16.0.0)
^1.2.3         compatible [1.2.3, 2.0.0)
~1.2.3         patch-level [1.2.3, 1.3.0)
>=1.0.0        greater or equal
>=1.0.0 <2.0.0 combined range
1.2.*          wildcard
1.0.0-beta.1   prerelease (sorts lower than 1.0.0)

Scope

• Replaces: catalog.cppm select_version_, resolver.cppm version selection, commands.cppm cmd_use latest sorting
• Unchanged: xvm internal version keys (gcc-15.1.0, glibc-2.39) — these are opaque binding labels, not user-facing versions

Test plan

• 28 unit tests (parse, compare, sort, range, select_best)
• xlings builds successfully
• Backward compatible: bare versions and prefix matching work as before