Summary

Adds an internal AbstractIL module (EncMethodDebugInformation) implementing, byte for byte, the three Portable PDB CustomDebugInformation blob formats Roslyn persists per method for Edit and Continue:

• EnC Local Slot Map (755F52A8-91C5-45BE-B4B8-209571E552BD)
• EnC Lambda and Closure Map (A643004C-0240-496F-A783-30D64F4979DE)
• EnC State Machine State Map (8B78CD68-2EDE-420B-980B-E15884B8AAA3)

with serializers, deserializers, a portable-PDB read-back helper, and an occurrence-key packing helper for deterministic syntax-offset slots. It also plumbs an optional methodCustomDebugInfoRows side channel through the IL binary writer options into the portable PDB generator, so a compilation can attach CDI rows to named methods.

Why

F# hot reload (#19941) needs Roslyn-compatible EnC method debug information in baseline PDBs so deltas can be chained across generations the same way C# Edit and Continue does. This PR carries the format layer only, so it can be reviewed as pure metadata/PDB code with deterministic byte evidence, independent of the hot reload feature.

There is intentionally no in-tree caller populating the side channel yet; the consumer lands with #19941. This follows the same upstreaming pattern as #20017: land isolated, test-covered infrastructure first, wire the feature in a later PR.

Behavior

• All existing writer call sites pass an empty map, so emitted PDBs are byte-identical to before (covered by a test asserting zero CustomDebugInformation rows for an empty map).
• Method names that do not identify exactly one IL method row are dropped (nothing attached); covered by absent-name and ambiguous-name tests.
• All additions are internal; no public surface area change.

Tests

17 tests in CompilerService/EncMethodDebugInformationTests.fs:

• blob round-trips for all three formats, including negative baselines, temps, ordinal-flagged slots, and negative state numbers;
• golden-byte encodings matching Roslyn's writer;
• cross-validation decoding and re-encoding, byte for byte, CDI blobs emitted by a real Roslyn (C#) compilation;
• fail-closed occurrence-key packing, including an int32-overflow regression where a wrapped negative key previously escaped the bound check;
• end-to-end synthetic PDB emission through the IL writer proving correct MethodDef parenting, zero rows for the empty map, and no rows for absent or ambiguous names.