Introduce analyzer to enforce proper HRESULT handling

docs/design/hresult-analyzer.md

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+**Analyzer goal:** prevent callers from accidentally ignoring the result of any method that returns the CsWin32 `HRESULT` type.
+# Scope
+
+Analyze every method invocation whose compile-time return type is the CsWin32 `HRESULT` type.
+
+The method does not need to be generated by CsWin32 or marked PreserveSig. This includes helper and wrapper methods that return an `HRESULT`.
+
+Exclude `HRESULT.ThrowOnFailure()` from analysis. Although it returns an `HRESULT`, it throws when the original value indicates failure. It can therefore return only a successful `HRESULT`, which does not require additional handling.
+# Recognized consumption
+
+1. Throw on failure: `obj.DoWork().ThrowOnFailure();`
+2. Test success: `if (obj.DoWork().Succeeded) { ... }`
+3. Test failure: `if (obj.DoWork().Failed) return;`
+4. Compare with an HRESULT: `if (hr == HRESULT.S_FALSE) { ... }`
+5. Switch on outcomes: `switch (hr) { case HRESULT.S_OK: ... }`
+6. Explicitly discard the result: `_ = obj.DoWork();`
+7. Pass the result to another method: `Log(obj.DoWork());`
+8. Store the result in a field or collection: `this.lastResult = obj.DoWork();`
+9. Return the result to the caller: `return obj.DoWork();`
+
+`PInvoke015` is conservative. It accepts any explicit use of the result because the caller did not accidentally ignore it.
+
+Reading `HRESULT.Value` does not by itself count as use. It transfers the tracking obligation to the resulting integer. Storing that integer in a local variable is insufficient until the value is read or otherwise used according to the same rules as an `HRESULT`.
+
+Some accepted uses have clearer alternatives:
+
+1. Prefer `hr.Failed` to `hr.Value < 0`.
+2. Prefer `hr.ThrowOnFailure()` to `Marshal.ThrowExceptionForHR(hr.Value)`.
+
+Use may occur through a local variable:
+
+```csharp
+HRESULT hr = obj.DoWork();
+
+if (hr.Failed)
+{
+    return;
+}
+```
+## Switch requirements
+
+Any switch that consumes the `HRESULT` counts as intentional use. For example:
+
+```csharp
+switch (hr)
+{
+    case HRESULT.S_OK:
+        UseResult();
+        break;
+
+    case HRESULT.S_FALSE:
+        HandleNoResult();
+        break;
+
+    default:
+        hr.ThrowOnFailure();
+        break;
+}
+```
+
+`PInvoke015` does not require the switch to distinguish a specific HRESULT or success/failure category. This is intentional to avoid excessively restricting handling patterns while still preventing consumers from accidentally ignoring HRESULTs.
+# Control-flow requirement
+
+Track each `HRESULT` through local variables. Every reachable path must consume the value before it is overwritten or the method exits.
+
+This is insufficient because the false path does not consume the HRESULT:
+
+```csharp
+HRESULT hr = obj.DoWork();
+
+if (condition)
+{
+    hr.ThrowOnFailure();
+}
+```
+
+# Diagnostic conditions
+
+Report `PInvoke015` if an HRESULT:
+
+1. Is ignored as an unused invocation result.
+2. Is assigned to a local that is never read.
+3. Is overwritten before its value is read.
+4. Has its `Value` stored in a local that is never read.
+5. Reaches a normal method exit without being consumed.
+6. Is not consumed on all reachable paths.
+# Insufficient handling
+
+```csharp
+obj.DoWork();
+HRESULT hr = obj.DoWork();        // Never checked
+int value = obj.DoWork().Value;   // Never used
+```
+
+Logging, storing outside a local, returning, or passing the HRESULT or its numeric value counts as intentional use and does not produce `PInvoke015`.
+# Propagation through wrappers
+
+Consider a wrapper around a CsWin32 method:
+
+```csharp
+HRESULT Initialize()
+{
+    return native.Initialize();
+}
+```
+
+Returning the HRESULT transfers responsibility. It does not handle the result.
+
+Because the analyzer checks every method that returns `HRESULT`, it also enforces handling at this call site:
+
+```csharp
+Initialize();
+```
+
+This supports transparent HRESULT wrappers without requiring explicit annotation.
+# Diagnostics
+
+| Rule         | Default severity | Condition                                         |
+| ------------ | ---------------- | ------------------------------------------------- |
+| `PInvoke015` | Warning          | HRESULT invocation result is accidentally ignored |
+# Analysis boundary
+
+Start analysis at every invocation whose compile-time return type is the CsWin32 `HRESULT` type, except `HRESULT.ThrowOnFailure()`. Track the value through local variables and control flow until every reachable path consumes it, overwrites it, or reaches a method exit.
+
+When analysis reaches an `HRESULT.Value` access, continue tracking the resulting integer. Apply the same control-flow and consumption rules to that integer as to the original `HRESULT`.