Align StreamReader with Ruby's IO conventions

[sribalakumar]

Fixes #149.

Opening this as a concrete proposal — happy to close it if you'd prefer the non-breaking route I mentioned in the issue (leave StreamReader as-is, add a separate IO-conformant class). No hard feelings either way.

What changed

StreamReader#read passed length straight through to the underlying IO, so it read that many compressed bytes and returned however many decompressed bytes fell out. The return size tracked the compression ratio instead of the caller's request.

This buffers decompressed output and serves reads from that buffer, so length means decompressed bytes.

	before	after
read(512), 44-byte frame	240000 bytes	512
read(512) ×4, 20 KB frame	[0, 0, 0, 0]	[512, 512, 512, 512]
at EOF	raises StandardError	nil
close	NoMethodError	closes the IO

Also adds eof?, read with no length, and an outbuf argument.

On decompress_with_pos

Refills use decompress_with_pos rather than decompress. decompress loops until all input is consumed, so output per call is unbounded — with a high compression ratio a 64 KB read can expand to hundreds of MB in the buffer. decompress_with_pos writes at most ZSTD_DStreamOutSize bytes per call and reports how much input it consumed, which keeps the buffer bounded regardless of ratio. That matters when the compressed input isn't trusted.

Measured on a 14.3 MB archive (64× ratio), peak buffer: decompress 4.6 MB → decompress_with_pos 0.57 MB.

Breaking change

read's semantics change, so this is breaking for anyone depending on the current behaviour. The existing spec's expectations change accordingly:

# before
expect(reader.read(10)).to eq('a')
expect(reader.read(10)).to eq('bcdef')
expect(reader.read(10)).to eq('')

# after
expect(reader.read(10)).to eq('abcdef')
expect(reader.read(10)).to be_nil

read also becomes arity-optional to match IO#read.

Tests

Full suite passes — 88 examples, 0 failures (was 69). New coverage: exact-length reads across block boundaries, ratio independence, nil at EOF, read with no args, read(0), negative length, outbuf (including the EOF case), eof?, close, custom chunk_size, round-trip integrity for both compressible and incompressible input, and an end-to-end test driving Gem::Package::TarReader through the reader.