Render diagnostics through the terminal library (#7683)

Diagnostic rendering was part of the data model: `Loc::FormatLocation`
and
`Loc::FormatSnippet` wrote plain text to a stream, so no capability,
color, or
terminal width could reach them.

A diagnostic is now one message with labels, contexts, and location
information
attached, and `Diagnostics::Renderer` draws it into a
`Terminal::Buffer`. The
design is in `toolchain/docs/diagnostics_rendering.md` (#7668),
authoring in
`toolchain/docs/diagnostics.md`.

Before:

```
example.carbon:6:3: error: 1 argument passed to function expecting 0 arguments
  Run0(1);
  ^~~~~~~
example.carbon:3:1: note: calling function declared here
fn Run0() {}
^~~~~~~~~~~
```

After:

```
error: 1 argument passed to function expecting 0 arguments
       ╭─┤ example.carbon:6:3
       ┆
     3 │ fn Run0() {}
       · ─────┬─────
       ·      ╰──┤ calling function declared here
       ┆
->   6 │   Run0(1);
       ·   ───────
       ╰────────────
```

`--no-diagnostic-snippets` gives a compact form, `example.carbon:6:3-9`.
File
tests use it; the rendering tests under `toolchain/diagnostics/testdata`
ask for
snippets.

C++ interop diagnostics are data now rather than text Clang drew:
`SemIR::ClangSourceLocId` names a `clang::SourceRange`, and a Clang
location is
marked as the point it names.

`StreamConsumer` moves to a `:renderer` target that `:emitter` does not
depend
on, keeping `common/terminal` out of `consumer.h`. Capabilities come
from the
file behind the error stream, so tests get plain ASCII.

`renderer_fuzzer` draws diagnostics built from fuzzer bytes with every
capability combination, since a shape the renderer doesn't expect must
never be
why the compiler dies while reporting a problem.

TODOs mark the follow-ups: line weights and crossing lines, wrapping
that keeps
a quoted type whole, and ordering diagnostics that share a byte offset.

Also: `IncorrectExtension`'s hint moves into its message, since its note
had no
location of its own, and `strip_noise` in `check_diagnostics.py` blanks
multi-line `#define`s instead of deleting them.

Assisted-by: Claude Code

toolchain/check/cpp/import.cpp

@@ -2071,9 +2071,9 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
   if (IsCppThunkRequired(context, callee_info)) {
     Diagnostics::AnnotationScope annotate_diagnostics(
         &context.emitter(), [&](auto& builder) {
-          CARBON_DIAGNOSTIC(InCppThunk, Note,
-                            "in thunk for C++ function used here");
-          builder.Note(loc_id, InCppThunk);
+          CARBON_DIAGNOSTIC_LABEL(InCppThunk, Info,
+                                  "in thunk for C++ function used here");
+          builder.Attach(loc_id, InCppThunk);
         });
     DefineAsThunkCall(context, loc_id, callee_info, imported_function);
   } else {
@@ -2138,9 +2138,10 @@ auto ImportFunctionPointerInvoke(
   }
   Diagnostics::AnnotationScope annotate_diagnostics(
       &context.emitter(), [&](auto& builder) {
-        CARBON_DIAGNOSTIC(InCppFunctionPointerThunk, Note,
-                          "in thunk for C++ function pointer type used here");
-        builder.Note(loc_id, InCppFunctionPointerThunk);
+        CARBON_DIAGNOSTIC_LABEL(
+            InCppFunctionPointerThunk, Info,
+            "in thunk for C++ function pointer type used here");
+        builder.Attach(loc_id, InCppFunctionPointerThunk);
       });
 
   CppCalleeFunctionInfo callee_info(context, info.clang_type);
@@ -2744,9 +2745,10 @@ auto ImportNameFromCpp(Context& context, SemIR::LocId loc_id,
     -> SemIR::ScopeLookupResult {
   Diagnostics::AnnotationScope annotate_diagnostics(
       &context.emitter(), [&](auto& builder) {
-        CARBON_DIAGNOSTIC(InCppNameLookup, Note,
-                          "in `Cpp` name lookup for `{0}`", SemIR::NameId);
-        builder.Note(loc_id, InCppNameLookup, name_id);
+        CARBON_DIAGNOSTIC_LABEL(InCppNameLookup, Info,
+                                "in `Cpp` name lookup for `{0}`",
+                                SemIR::NameId);
+        builder.Attach(loc_id, InCppNameLookup, name_id);
       });
   if (IsIncompleteClass(context, scope_id)) {
     return SemIR::ScopeLookupResult::MakeError();