feat: close iOS/Android platform parity gaps

[cursor]

Motivation

The README platform matrix documents several iOS gaps that force app authors into platform-specific branches:

Gap	Android	iOS today
AES for zipWithPassword(string[])	✅	❌ STANDARD only
Compression level for file arrays	✅	ignored
Charset on unzip / getUncompressedSize	✅	ignored (UTF-8 only)
unzipAssets / bundle resource unzip	✅ assets + content://	❌
Empty directories preserved in zip(string[])	✅	❌

Proposal

Close these gaps where the underlying libraries allow:

1. iOS AES for file-array password zips — match folder AES behavior
2. iOS compression level for file arrays — honor 0–9 / constants
3. iOS charset — honor charset where feasible, or document a hard limitation with a clear error if unsupported
4. iOS bundle unzip — unzipAssets equivalent for app bundle / main-bundle resources
5. Empty directory preservation — include empty dirs when zipping directory array items on iOS

Acceptance criteria

• Platform support matrix updated; ⚠️ cells reduced
• Each item above has iOS + Android playground / E2E coverage where practical
• No regressions to existing folder-based zip/unzip APIs

Priority

P1