Throw IOException for invalid or unsupported frame descriptors

[yawkat]

Fixes #102.

LZ4FrameInputStream.readHeader() called FLG.fromByte/BD.fromByte unguarded. For reserved bits, dependent blocks, a dictionary ID, a bad version or block-size indicators 0-3, those calls throw a plain RuntimeException/IllegalArgumentException. That exception escaped from read(), skip() and the content-size getters. Both calls are now wrapped, and failures are rethrown as IOException("Invalid or unsupported frame descriptor", cause). The public FLG/BD API is unchanged.

Note: descriptor validation still runs before the header checksum check. A bit flip in FLG or BD is therefore reported as invalid/unsupported rather than as a checksum mismatch; either way it is now an IOException.

Test: testInvalidOrUnsupportedFrameDescriptor builds headers with valid checksums for FLG 0x40, 0x61, 0x62, 0x20 and BD 0x30, 0x41. For each it asserts IOException with that message from read(), skip(), getExpectedContentSize() and isExpectedContentSizeDefined(). It fails without the fix.

Note: this touches the same file as #125 and #131. Whichever is merged later may need a trivial rebase.

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.44%. Comparing base (f6e3304) to head (debfd38).
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Additional details and impacted files

@@             Coverage Diff              @@
##               main     #133      +/-   ##
============================================
+ Coverage     79.76%   80.44%   +0.68%     
  Complexity      562      562              
============================================
  Files            42       42              
  Lines          1838     1841       +3     
  Branches        247      247              
============================================
+ Hits           1466     1481      +15     
+ Misses          239      233       -6     
+ Partials        133      127       -6     

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