[Repo Assist] Perf: cache member wrapper objects in TargetTypeDefinition

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Problem

TargetTypeDefinition.GetConstructors / GetMethods / GetFields / GetEvents / GetProperties / GetNestedTypes recreated all wrapper objects (anonymous ConstructorInfo, MethodInfo, FieldInfo, PropertyInfo, EventInfo, nested Type instances) on every call:

// Before – wrapper objects created fresh each call
override this.GetMethods(bindingFlags) =
    inp.Methods.Entries
    |> Array.filter (fun x -> x.Name <> ".ctor" && x.Name <> ".cctor")
    |> Array.map (txILMethodDef this)           // ← new anonymous objects every time
    |> Array.filter (canBindMethod bindingFlags)

When the F# compiler queries the same target type more than once during a compilation session (which it does for types used as base classes, interface types, generic constraints, etc.), the wrappers were silently discarded and rebuilt each call — O(n) allocation with no benefit.

Fix

Introduce six lazy caches (ctorDefs, methDefs, fieldDefs, eventDefs, propDefs, nestedDefs) so wrapper construction happens once per TargetTypeDefinition instance. The BindingFlags filter is still applied on every call (cheap) while the expensive traversal and object allocation is amortised:

// After – wrapper objects created once per type
let methDefs = lazy (inp.Methods.Entries
                     |> Array.filter (fun x -> x.Name <> ".ctor" && x.Name <> ".cctor")
                     |> Array.map (txILMethodDef this))

override __.GetMethods(bindingFlags) =
    methDefs.Force() |> Array.filter (canBindMethod bindingFlags)
```

`TargetTypeDefinition` instances are themselves created once per type token (via `TxTable.Get inp.Token`), so the lazy values live as long as the type object does — no premature GC, no stale state.

Thread safety is provided by the default `Lazy(T)` publication mode (`ExecutionAndPublication`).

## Impact

For generative type providers that use Framework types as reference contexts (e.g. for base classes or attribute constructors), the compiler may call `GetMethods` on `TargetTypeDefinition` instances wrapping large types (like `System.Object`, `System.Enum`, attribute types). Previously every such call rebuilt all wrappers from scratch. This change avoids the redundant allocation.

## Test Status

```
Passed!  - Failed: 0, Passed: 110, Skipped: 0, Total: 110, Duration: 6 s

All 110 existing tests pass unchanged.

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