`NullReferenceException` breaks IDE syntax coloring on unlinked placeholder entities (e.g. broken code mid-edit)

**Succinct description:**
NullReferenceException (NRE) in `TypedTree` during semantic classification crashes **entire document coloring** in F# IDE/LSP/VS Code/VS.

**Root cause:** Mid-edit broken/unresolved symbols → `null` `entity_modul_type` / `Entity` (unlinked placeholders). Derefs like `entity_modul_type.Force()` / `entity.DisplayName` → NRE → full pipeline crash.

**Fix ready in staged changes:** Defensive null-guards + comments in `TypedTree.Construct.NewModified*`, `FreeVars` etc. Partial classification survives.
See [detailed doc](NullRef-TypedTree-Classification-Fix.md), [#20269](https://github.com/dotnet/fsharp/issues/20269).

**Repro steps**

1. VS/VS Code: New F# file (`*.fs`).
```
type Foo = // Delete 'type' → unresolved
member x.Bar = ()
```
2. Save → trigger typecheck/classification (edit/save).
3. **Observe:** **Entire file loses semantic colors** (keywords gray, no blue types/keywords).

Pre-fix: NRE in VS Output (F#) / VS Code DevTools Console (LSP).

**Expected behavior**

Partial/resilient classification:
- Valid code: Full semantic colors (blue types, green keywords).
- Broken symbol: Isolated error (gray/underline), rest intact.

**Actual behavior**

NRE propagates:
```
TypedTree.fs / TypedTreeOps.Remapping.fs → entity_modul_type.Force() → NRE
↑
LanguageService.fs → DocumentCache.fs → classification crash
↓
LSP: semanticTokens/full → empty → no colors for whole file
```
UX: Plain syntax-only text mid-edit.

**Known workarounds**

- Fix broken symbol immediately.
- Restart IDE / Reload VS Code window (`Ctrl+Shift+P` > "Developer: Reload Window").
- Disable semantic highlighting (VS Code: `fsharp.semanticHighlighting.enabled: false`).

**Related information**

* **OS:** Windows (tested; likely cross-platform).
* **.NET Runtime:** net9.0 (F# compiler).
* **Editing Tools:**
- Visual Studio 17.12+ (F# workload).
- VS Code + F# extension (LSP).

**Staged fix files** (`git diff --staged`):
```
M src/Compiler/TypedTree/TypedTree.fs // Null-guards + comments
M src/Compiler/TypedTree/TypedTree.fsi // Sigs
M src/Compiler/TypedTree/TypedTreeOps.Remapping.fs
M src/Compiler/Utilities/lib.fs
M vsintegration/src/FSharp.Editor/* // LS/cache resilience
```

**Validation:** `./build.sh -c Release --testcoreclr` clean. IDE resilient.
**Commit:** `Fix NRE in TypedTree: resilient IDE classification (#20269)`

Start with the staged changes in src/Compiler/TypedTree/TypedTree.fs, TypedTree.fsi, TypedTreeOps.Remapping.fs, Utilities/lib.fs, and the vsintegration/src/FSharp.Editor files, then reproduce the broken F# file and inspect the classification path. Run ./build.sh -c Release --testcoreclr; done means broken symbols no longer remove coloring from the rest of the document and the validation remains clean.

Written by the indexing model from the issue text.