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/handle_binding_pattern.cpp

@@ -101,9 +101,9 @@ static auto IsValidParamForIntroducer(Context& context, SemIR::LocId loc_id,
         auto builder =
             context.emitter().Build(loc_id, GenericParamMustBeConstant);
         if (is_var) {
-          CARBON_DIAGNOSTIC(VarParamIsRuntime, Note,
-                            "`var` parameters are runtime");
-          builder.Note(loc_id, VarParamIsRuntime);
+          CARBON_DIAGNOSTIC_LABEL(VarParamIsRuntime, Info,
+                                  "`var` parameters are runtime");
+          builder.Attach(loc_id, VarParamIsRuntime);
         }
         builder.Emit();
         return false;
@@ -372,12 +372,12 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
   };
 
   auto abstract_diagnostic_context = [&](auto& builder) {
-    CARBON_DIAGNOSTIC(AbstractTypeInVarPattern, Context,
-                      "binding pattern has abstract type {0} in `var` "
-                      "pattern",
-                      SemIR::TypeId);
-    builder.Context(type_expr.node_id, AbstractTypeInVarPattern,
-                    type_expr.type_component_id);
+    CARBON_DIAGNOSTIC_CONTEXT(AbstractTypeInVarPattern,
+                              "binding pattern has abstract type {0} in `var` "
+                              "pattern",
+                              SemIR::TypeId);
+    builder.Attach(type_expr.node_id, AbstractTypeInVarPattern,
+                   type_expr.type_component_id);
   };
 
   // A `self` binding must be the first parameter in the explicit parameter
@@ -516,12 +516,13 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
         return context.TODO(node_id, "support local form bindings");
       }
       auto incomplete_diagnostic_context = [&](auto& builder) {
-        CARBON_DIAGNOSTIC(IncompleteTypeInBindingDecl, Context,
-                          "binding pattern has incomplete type {0} in name "
-                          "binding declaration",
-                          InstIdAsType);
-        builder.Context(type_expr.node_id, IncompleteTypeInBindingDecl,
-                        type_expr.inst_id);
+        CARBON_DIAGNOSTIC_CONTEXT(
+            IncompleteTypeInBindingDecl,
+            "binding pattern has incomplete type {0} in name "
+            "binding declaration",
+            InstIdAsType);
+        builder.Attach(type_expr.node_id, IncompleteTypeInBindingDecl,
+                       type_expr.inst_id);
       };
       if (node_kind == Parse::NodeKind::VarBindingPattern) {
         if (!RequireConcreteType(