Add ECMA-335 EnC metadata delta writer

[NatElkins]

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.

Sequencing

This PR is part of splitting the F# hot reload work (#19941) into small, independently reviewable PRs. The planned order:

1. Wave 1 (independent of each other, no cross dependencies): Add ResetCompilerGeneratedNameState to compiler-generated name generators #20017 (generated-name counter reset), Add Roslyn-format EnC CustomDebugInformation codec and portable PDB method CDI emission #20018 (EnC CustomDebugInformation codec and method CDI emission), Add ECMA-335 EnC metadata delta writer #20019 (EnC metadata delta writer), plus two upcoming slices: stable synthesized-name replay and the typed-tree differ with rude-edit classification.
2. Wave 2: baseline reading and recorded CDI state (depends on Add Roslyn-format EnC CustomDebugInformation codec and portable PDB method CDI emission #20018 and the naming slice).
3. Wave 3: the delta emitter and symbol matcher.
4. Wave 4: the hot reload session, FCS surface, and the --test:HotReloadDeltas capture hook (F# hot reload: Edit-and-Continue delta emission behind --test:HotReloadDeltas #19941 in its final, much smaller form).
5. Last, explicitly experimental: Add an experimental flag-gated in-process compile path for hot reload sessions #20031 (the flag-gated in-process compile perf path).

Each slice is rebased on current main, carries its own tests, and is flag-off zero-cost. Everything stays draft until reviewed in order.
Role in the train: the hot reload session emits its metadata deltas through this writer.

Refresh status (2026-07-17)

• Refreshed against dotnet/fsharp main at 5928e91; the current reviewed head is e7d79c0.
• A dedicated review pass completed for this PR, its findings were fixed in the lowest owning slice, and the PR has no unresolved review threads.
• The downstream session, in-process compiler, umbrella, and SDK branches were restacked after the fixes, so this slice remains part of the decomposed review train.
• The complete compiler stack passed the 11-step hot reload verifier, 456 service tests, 243 component tests with 2 expected skips, and 1411 EmittedIL tests with 3 expected skips. Replacement CI passed on this exact head.

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

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