Add missing portable PDB debug data types and fix the FPO value

[Sergio0694]

Summary

Adds the two portable PDB debug data types that DebugDataType was missing, and fixes Fpo, which was defined as 0 instead of 3.

Motivation

DebugDataType has no members for two of the debug directory entry types that every deterministic .NET assembly built with embedded symbols carries:

Constant	Value	Status
IMAGE_DEBUG_TYPE_EMBEDDED_PORTABLE_PDB	17	missing
IMAGE_DEBUG_TYPE_PDBCHECKSUM	19	missing

Both are defined by the Portable PDB specification and are present in System.Reflection.PortableExecutable.DebugDirectoryEntryType. Reading and writing these entries already works (they fall through to CustomDebugDataSegment in DefaultDebugDataReader), but callers have to cast a magic number to name them:

// Today
image.DebugData.Add(new DebugDataEntry(new CustomDebugDataSegment((DebugDataType)17, pdb)));

// With this change
image.DebugData.Add(new DebugDataEntry(new CustomDebugDataSegment(DebugDataType.EmbeddedPortablePdb, pdb)));

Separately, Fpo is declared as 0, but IMAGE_DEBUG_TYPE_FPO is 3. As a result DebugDataType.Fpo == DebugDataType.Unknown is true, and a real type 3 entry is surfaced as an unnamed (DebugDataType)3 instead of Fpo. This looks like a typo rather than something intentional, since every other member matches its winnt.h value.

Changes

• src/AsmResolver.PE/Debug/DebugDataType.cs: adds EmbeddedPortablePdb = 17 and PdbChecksum = 19, and corrects Fpo from 0 to 3.

• test/AsmResolver.PE.Tests/Debug/DebugDataEntryTest.cs: adds PersistentPortablePdbEntries, asserting that entries of the newly named types survive a build and reload round trip alongside Repro.

Notes

Changing the value of Fpo is technically a source and binary compatible but behavior changing fix, so it is worth calling out explicitly. In practice nothing can be relying on the current value in a meaningful way: it is indistinguishable from Unknown, so any code branching on DebugDataType.Fpo today is matching unknown entries rather than FPO entries. I am happy to split it into its own pull request if you would rather keep the additions separate from the fix.

18 is deliberately not added: unlike 17 and 19 it is not part of the specification linked above and has no winnt.h constant.

All 313 tests in AsmResolver.PE.Tests pass.

[Washi1337]

LGTM, if you can rebase the branch to latest dev then I can merge it safely.