release: 0.3.0 — fetch-first workflow

[N283T]

Summary

• Bump version to 0.3.0 in build.zig.zon and flake.nix
• Update CHANGELOG with 0.3.0 release notes

[0.3.0] - 2026-04-16

Removed

• Embedded PDBx dictionary (@embedFile) — mmcif-dict fetch is now required
  on first use; binary shrinks from ~3 MB to ~1 MB (refactor: remove @embedFile, switch to fetch-first workflow #34)
• Two-stage build (compile_tool host binary and data/mmcif_pdbx.dic)

Changed

• Dictionary resolution falls through to a clear error message instead of an
  embedded fallback when no cache is found