Speed up reading packages and fix some bugs

[w1tcherrr]

Changes

• Archive files are opened and memory mapped once per package. GetMemoryMappedStreamIfPossible returns a lightweight stream over that mapping. ReadEntry reads with RandomAccess from the cached handles.
• Both are now thread-safe.
• The directory tree is parsed from a single buffer. Sorting is unchanged.
• Verification reads into reused buffers.

Benchmark

Benchmark	Unit	Before	After
Open CS2 pak01_dir.vpk	ms	50.6	27.9
Open Dota 2 pak01_dir.vpk	ms	210.6	101.7
Mapped stream + read, entries up to 4 KiB	us	36.70	1.13
Mapped stream + read, entries of 16-256 KiB	us	44.10	3.33
Mapped stream + read, entries of 4 MiB or more	us	3137	321
ReadEntry, entries up to 4 KiB	us	37.4	3.7
Mapped reads on 16 threads, entries up to 4 KiB	reads/s	25,300	7,890,000
VerifyChunkHashes, CS2 archives 0-5	ms	1358	626
VRF loading 7,397 CS2 resources, 1 thread	ms	544	320
VRF loading 7,397 CS2 resources, 16 threads	ms	283	203

Fixes

• Concurrent ReadEntry calls could return wrong data.
• Entries cut off by the end of an archive were returned zero-filled. They now throw EndOfStreamException.
• Calling Read again kept the previous package's signature and checksums, and IsDirVPK never reset in SetFileName.
• AddFile on a optimized package appended unsorted, so FindEntry missed the file.
• Read(Stream) and Write(Stream) broke when the package did not start at position 0, and writing into a longer stream hashed the leftover bytes.
• Malformed signature and archive MD5 section sizes now throw InvalidDataException.

Behaviour changes

• GetMemoryMappedStreamIfPossible returns an internal stream type instead of MemoryMappedViewStream. Returned streams must not be disposed while another thread reads them.
• Archive files stay open and mapped until the package is disposed, and chunk files use FileShare.ReadWrite.
• ReadEntry, GetMemoryMappedStreamIfPossible and the Verify* methods throw ObjectDisposedException after Dispose.

I ran this PR over hundreds of different game paks and they read identical to before.