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/decl_name_stack.cpp

@@ -393,11 +393,11 @@ static auto DiagnoseQualifiedDeclInIncompleteClassScope(Context& context,
     -> void {
   Diagnostics::ContextScope diagnostic_context(
       &context.emitter(), [&](auto& builder) {
-        CARBON_DIAGNOSTIC(QualifiedDeclInIncompleteClassScope, Context,
-                          "cannot declare a member of incomplete class {0}",
-                          SemIR::TypeId);
-        builder.Context(loc_id, QualifiedDeclInIncompleteClassScope,
-                        context.classes().Get(class_id).self_type_id);
+        CARBON_DIAGNOSTIC_CONTEXT(
+            QualifiedDeclInIncompleteClassScope,
+            "cannot declare a member of incomplete class {0}", SemIR::TypeId);
+        builder.Attach(loc_id, QualifiedDeclInIncompleteClassScope,
+                       context.classes().Get(class_id).self_type_id);
       });
   DiagnoseIncompleteClass(context, class_id);
 }
@@ -409,11 +409,11 @@ static auto DiagnoseQualifiedDeclInUndefinedInterfaceScope(
     SemIR::InstId interface_inst_id) -> void {
   Diagnostics::ContextScope diagnostic_context(
       &context.emitter(), [&](auto& builder) {
-        CARBON_DIAGNOSTIC(QualifiedDeclInUndefinedInterfaceScope, Context,
-                          "cannot declare a member of undefined interface {0}",
-                          InstIdAsType);
-        builder.Context(loc_id, QualifiedDeclInUndefinedInterfaceScope,
-                        interface_inst_id);
+        CARBON_DIAGNOSTIC_CONTEXT(
+            QualifiedDeclInUndefinedInterfaceScope,
+            "cannot declare a member of undefined interface {0}", InstIdAsType);
+        builder.Attach(loc_id, QualifiedDeclInUndefinedInterfaceScope,
+                       interface_inst_id);
       });
   DiagnoseIncompleteInterface(context, interface_id);
 }
@@ -426,11 +426,11 @@ static auto DiagnoseQualifiedDeclInImportedPackage(Context& context,
     -> void {
   CARBON_DIAGNOSTIC(QualifiedDeclOutsidePackage, Error,
                     "imported packages cannot be used for declarations");
-  CARBON_DIAGNOSTIC(QualifiedDeclOutsidePackageSource, Note,
-                    "package imported here");
+  CARBON_DIAGNOSTIC_LABEL(QualifiedDeclOutsidePackageSource, Info,
+                          "package imported here");
   context.emitter()
       .Build(use_loc_id, QualifiedDeclOutsidePackage)
-      .Note(import_loc_id, QualifiedDeclOutsidePackageSource)
+      .Attach(import_loc_id, QualifiedDeclOutsidePackageSource)
       .Emit();
 }
 
@@ -442,11 +442,11 @@ static auto DiagnoseQualifiedDeclInNonScope(
   CARBON_DIAGNOSTIC(QualifiedNameInNonScope, Error,
                     "name qualifiers are only allowed for entities that "
                     "provide a scope");
-  CARBON_DIAGNOSTIC(QualifiedNameNonScopeEntity, Note,
-                    "referenced non-scope entity declared here");
+  CARBON_DIAGNOSTIC_LABEL(QualifiedNameNonScopeEntity, Info,
+                          "referenced non-scope entity declared here");
   context.emitter()
       .Build(use_loc_id, QualifiedNameInNonScope)
-      .Note(non_scope_entity_loc_id, QualifiedNameNonScopeEntity)
+      .Attach(non_scope_entity_loc_id, QualifiedNameNonScopeEntity)
       .Emit();
 }