fix: compare_version handles pre-release suffixes (closes #98)

[avrabe]

Summary

Fixes the cosmetic correctness bug in meld-core/src/merger.rs::compare_version reported in #98.

The previous implementation parsed each dot segment with filter_map(parse::<u32>), which silently dropped any segment containing non-numeric characters. As a result compare_version("0.2.99-rc1", "0.2.0") returned Ordering::Less — the "99-rc1" segment was discarded, leaving [0, 2] vs [0, 2, 0].

This PR replaces the body with a small inline implementation of the relevant subset of semver 2.0.0 precedence rules (no new dependency, ~70 LOC):

• Build metadata (+...) is stripped before comparison.
• The MAJOR.MINOR.PATCH triple is compared numerically; missing trailing segments default to 0 ("0.2" == "0.2.0").
• A version with a pre-release suffix sorts below the same version without one (0.2.0-rc1 < 0.2.0).
• Pre-release identifiers are compared dot-segment-wise: numeric ids numerically, alphanumerics lexically, numeric ids below alphanumerics (semver §11.4.3).
• Non-numeric main segments fall back to a lexical compare of that segment instead of being silently dropped.

The function signature (a: &str, b: &str) -> std::cmp::Ordering is unchanged.

Test plan

Eight new unit tests in meld-core/src/merger.rs:

• test_compare_version — pure numeric triples (existing coverage)
• test_compare_version_prerelease_regression_issue_98 — the exact reproducer from the issue
• test_compare_version_prerelease_below_release — 0.2.0-rc1 < 0.2.0
• test_compare_version_prerelease_ordering — alpha < beta, rc1 < rc2, numeric < alphanumeric, longer pre-release wins
• test_compare_version_build_metadata_ignored — +meta does not affect precedence (with or without pre-release)
• test_compare_version_missing_segments — "0.2" == "0.2.0", "1" == "1.0.0"
• test_compare_version_mixed_alphanumeric — non-numeric main segments are no longer silently dropped
• test_compare_version_large_numbers — 0.0.10 > 0.0.9 (numeric, not lexical)

Verified locally:

• cargo fmt --check clean
• cargo clippy --package meld-core --all-targets clean
• cargo test --release passes (incl. the 73-test wit_bindgen_runtime suite, 30 merger unit tests)
• Pre-commit hook chain (fmt + clippy + test) passes

Closes #98.

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