Summary

Adds an internal, standalone ECMA-335 Edit-and-Continue metadata delta writer to AbstractIL, with focused System.Reflection.Metadata-readable metadata tests. It has no behavior change and no in-tree semantic consumer yet; the consumer is the F# hot reload work in #19941. This follows the same upstreaming pattern as #20017 and #20018: land isolated infrastructure first, with deterministic byte and layout evidence, then wire the feature in a later PR.

What

Ten new internal AbstractIL files (file names kept from the development branch for diffability):

• ILDeltaHandles - offset/handle primitives, coded-index discriminated unions, delta tokens
• ILMetadataHeaps - heap sizes and offset bookkeeping
• IlxDeltaStreams - user-string and standalone-signature token calculators (continue numbering from a baseline)
• DeltaMetadataEncoding - ECMA-335 II.24.2.6 coded-index encoding
• DeltaMetadataTypes - the writer's input model: plain row-description records (method/parameter/property/event rows, method-semantics updates, custom attribute rows) and the definition key types
• DeltaMetadataTables / DeltaTableLayout / DeltaIndexSizing / DeltaMetadataSerializer - delta #- table stream, heap construction, layout and index sizing, EncLog/EncMap emission, generation GUID chaining
• FSharpDeltaMetadataWriter - the coordinating writer

The writer's inputs are row records (names, tokens, signature blobs, RVAs) plus baseline heap offsets and row counts. It imports nothing from any diffing, baseline, or session machinery; a grep gate for such imports is part of how the extraction was validated. One line of ilwrite.fsi exposes the pre-existing markerForUnicodeBytes so the delta writer reuses the full writer's exact string-marker logic.

Tests

130 tests in tests/FSharp.Compiler.Service.Tests/DeltaMetadata/:

• coded-index encodings asserted against the production Encoding.CodedIndices definitions, with ECMA-335 II.24.2.6 table order cross-checked against System.Reflection.Metadata's TableIndex as an independent reference;
• System.Reflection.Metadata reader parity over emitted deltas (SrmReaderParityTests);
• EncLog/EncMap correctness, stream layout, heap and index sizing;
• multi-generation chaining with generation-2 heap offsets asserted against computed expected values;
• standalone-signature delta rows asserted at baseline + 1 from a real seeded baseline;
• serializer failure paths.

All additions are internal; no public surface area change.