fix: ASCII85 decoder drops 'z' zero-groups and never signals EOF at ~> marker

[chappihappymeal]

Fixes #83.

alphaReader (the sanitizing reader in front of encoding/ascii85) had two bugs:

1. It stripped z — the standard ASCII85 shorthand for an all-zero 4-byte group (PDF 32000-1:2008, §7.4.3), emitted by Ghostscript, Adobe tools, and Go's own ascii85.Encode. Every z silently dropped 4 bytes from the decoded stream: with [/ASCII85Decode /FlateDecode] this corrupted the flate stream (GetPlainText() → malformed PDF: unexpected EOF, Page.Content() → panic escaping to the caller); with plain /ASCII85Decode the corruption was silent.
2. On the ~> end-of-data marker it only breaked out of the sanitizing loop and returned n, nil — it never signaled io.EOF, never truncated at the marker, and kept feeding post-marker bytes from later Read chunks to the decoder.

The rewritten Read filters in place (no per-call allocation), passes z through, and returns the sanitized byte count with io.EOF as soon as ~ is seen — per spec ~ can only start the EOD marker, which also handles a marker split across two reads for free. After that every Read returns 0, io.EOF without touching the underlying reader.

Tests: 6 unit tests for alphaReader (zero-group, EOF at marker, marker split across reads, garbage after marker, line-wrapped input, reads after EOF) and 2 integration tests with minimal synthetic PDFs in testdata/ — one with a z group inside a text string (plain /ASCII85Decode), one with an Exstream-style [/ASCII85Decode /FlateDecode] chain. Both fixtures are read correctly by poppler's pdftotext and Python's base64.a85decode, and both failed on the old code.

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On a separate note: I noticed several open PRs fixing hangs/OOMs (#46, #58, #64, #76, #78, #79) have been waiting for review for a while. I understand maintaining this alone is a lot. I use this library in production for high-volume PDF processing (bank receipts, dozens of layouts) and have a large regression corpus for it — I'd be happy to help with triage and reviews as a co-maintainer if that's useful to you. If you'd rather keep it as is, no problem at all — feel free to just take the fix.