feat: add ZIP64 archive support (validation and fixtures) #63

Summary

Validate and, if needed, fix extraction of ZIP64 archives (>4 GB or >65,535 entries). Required for 1.0 by the spec.

Motivation

Large game/backup archives use ZIP64; support is currently untested.

Requirements

• Generated fixtures in tests (large sparse entries, many entries) within CI time limits.
• Update docs/archive-support.md with the observed result.

Controller UX

No new UX.

Technical Notes

SharpCompress claims ZIP64 read support; verify through SafeExtractor limits and CRC checks.

Acceptance Criteria

• A ZIP64 fixture extracts with correct content and sizes on the Windows runner.

Testing

Unit tests for new branches and for every bug reproduced before a fix (no trivial variations, see AGENTS.md).

Dependencies

• None

Priority: medium · Milestone: 6 · Archives