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/export.cpp

@@ -1647,12 +1647,13 @@ auto ExportFunctionToCppPointerConversion(
           ExportedFunctionPtrTypeMismatch, Error,
           "can't convert exported {0:member |}function pointer to `{1}`",
           Diagnostics::BoolAsSelect, CppType);
-      CARBON_DIAGNOSTIC(ExportedFromFunction, Note,
-                        "function exported with pointer type `{0}`", CppType);
+      CARBON_DIAGNOSTIC_LABEL(ExportedFromFunction, Info,
+                              "function exported with pointer type `{0}`",
+                              CppType);
       context.emitter()
           .Build(src_id, ExportedFunctionPtrTypeMismatch,
                  src_ptr_type->isMemberFunctionPointerType(), dest_ptr_type)
-          .Note(function.first_decl_id(), ExportedFromFunction, src_ptr_type)
+          .Attach(function.first_decl_id(), ExportedFromFunction, src_ptr_type)
           .Emit();
     }
     return false;