Add typed-tree differ and edit classification for hot reload

[NatElkins]

Adds an internal typed-tree differ for F# hot reload: TypedTreeDiff compares two CheckedImplFiles and classifies each change as a semantic edit (method body update, insert) or a rude edit (declaration removed, signature change, type-parameter constraint change, inline-info change, type layout change). Reference-equal files short-circuit to an empty diff, which is the sound fast path when the transparent compiler returns unchanged files.

This is the classification layer the hot reload session uses to decide what goes into an Edit and Continue delta and what must fall back to a restart. It is pure: no session state, no runtime capability handling, no emission. Entity logical names carry generic arity where the typed tree provides it, so generic and non-generic types stay distinct in the diff.

Nothing on main calls this yet; the module is internal and the surface area baseline is untouched. Runtime capability negotiation and the deeper lambda and state machine occurrence classification stay in the later session slice, where their dependencies actually live.

Tests: 8 unit tests covering the fast path, body edits, inserts, removals, signature and constraint changes, and layout changes. SurfaceAreaTest green.

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), Add stable synthesized-name replay infrastructure for hot reload #20024 (stable synthesized-name replay), and this PR.
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 Add stable synthesized-name replay infrastructure for hot reload #20024).
3. Wave 3: the delta emitter and symbol matcher (consumes this differ).
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).

Role in the train: the delta emitter (wave 3) consumes this diff to scope emission; the session (wave 4) consumes the rude-edit classification to decide restart versus apply.

Refresh status (2026-07-17)

• Refreshed against dotnet/fsharp main at 5928e91; the current reviewed head is 97ab0e6.
• 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