fix: Support two-space list indentation tolerance

[Fuyeors]

Summary

This change supports two-space-step list nesting while retaining legacy odd-space indentation compatibility. Nested list markers are normalized before recursive block parsing, and regression coverage now includes 2-space, 3-space, and 4-space nesting.

Validation

The following local checks pass:

Check	Result
pnpm --filter @fuyeor/markdown-parser typecheck	Pass
pnpm --filter @fuyeor/markdown-parser test:unit	11/11 pass
pnpm --filter @fuyeor/markdown-parser test:ffm	10 pass, 458 skipped by existing compatibility filters
Prettier and git diff --check	Pass

Performance and package size

The benchmark used identical Node/Vitest settings and compared against origin/main. Five runs were used for the flat and new nested scenarios; three runs were used for the legacy nested scenario.

Scenario	Median parse time	Median sampled peak heap
Flat lists	4.0065 ms → 3.9783 ms (-0.7%)	10.88 MiB → 6.24 MiB (-42.6%)
Legacy 3/5-space nested lists	10.7208 ms → 10.8198 ms (+0.9%)	10.07 MiB → 5.75 MiB (-42.9%)
New 2/4-space nested lists	7.2906 ms → 10.7815 ms (+47.9%)	12.98 MiB → 9.72 MiB (-25.1%)

The new 2/4-space nested input now produces the requested nested AST, so its additional work reflects the newly enabled structure rather than a regression in the legacy-compatible path.

The npm package tarball increased from 43,063 to 43,647 bytes (+584 bytes, +1.36%); unpacked size increased by 2,214 bytes (+0.76%).

CI note

The first GitHub Actions check currently fails during pnpm setup because the existing workflow requests pnpm 11.20.0 while the repository packageManager field requests pnpm 11.22.0. The failure occurs before dependency installation and before any project test runs; it is unrelated to this change.