Honour LZWDecode /EarlyChange 0; expand filter test coverage

[fank]

Bug

decodeLZW interpreted the early-change flag as:

early := p.EarlyChange
if early == 0 { early = 1 }   // <- explicit /EarlyChange 0 becomes 1

/EarlyChange defaults to 1 but may be set to 0. Because the field's zero value (0) doubled as "unset", a stream that explicitly declared /EarlyChange 0 was silently decoded with early change on, producing garbage past the first 9→10-bit code-width boundary (~511 dict entries).

Fix

Replace Params.EarlyChange int with Params.NoEarlyChange bool:

• zero value → early change on (the PDF default), correct for the common case;
• explicit /EarlyChange 0 → NoEarlyChange = true.

As a bonus this clamps the effective flag to {0,1}, so a hostile /EarlyChange value (e.g. a huge int) can no longer drive the width threshold (1<<width) - early negative.

How the oracle works

Go's compress/lzw (MSB) turned out to use the non-early convention, so its output round-trips this decoder only with NoEarlyChange: true. That makes it a sound independent oracle for the early=0 path and the shared width-growth / clear / KwKwK machinery. The red→green was verified by temporarily reverting the fix (the /EarlyChange 0 stream then fails with invalid code … at width 9).

Tests

• TestLZWRoundTripStdlib — stdlib-encoded streams (incl. a 64 KiB varied buffer) exercise dict reuse, KwKwK, 9→12-bit growth and the dictionary-full reset.
• TestLZWEarlyChangeHonored / TestLZWStreamEarlyChangeZero — the flag is honoured, the latter at the Open()→Content() surface (also covers paramsFromDict + streamFilterChain).
• TestLZWTruncatedNoPanic — a stream cut mid-code must not panic (readBits zero-padding).
• TestASCII85RoundTripStdlib / TestASCII85EdgeCases — partial groups, z, whitespace, <~, invalid byte.
• TestStreamFilterChainArray — a chained [/ASCII85Decode /FlateDecode] filter.

Also removes the dead errors import (var _ = errors.New) from lzw.go.

Coverage: internal/filter 80.7% → 93.2%; decodeLZW, readBits, decodeASCII85 → 100%; paramsFromDict 0% → 67%; streamFilterChain 42% → 67%.