json_view: address review of the exception diagnostics

- value_path(): check for the empty stack where the last frame is popped,
  so that the loop is `for (;;)`; update the frame only after the target
  check; document that the frame reference is not used after pop_back()
  or push_back() (and add a test with deep documents to run under a
  sanitizer)
- value_range(): flip the container check to remove a level of nesting
- diagnostics(): wrap the whole function in the macro check, and compute
  the path before the bytes, so that nothing is computed when there is no
  path; the function stays a template
- error_site: separate constructors for the two configurations
- site_of(): static_assert that it is called with a view
- document why the throw helpers without an error_site have none
- pointer.hpp: do not split a comment around the template header
- docs: "when materialize() sets them"; list the effects of
  JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS separately

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasBf4ToHzdcoNEPAJhS

docs/mkdocs/docs/api/basic_json_view/value.md

@@ -63,7 +63,7 @@ Strong guarantee: there are no changes to the view or the document it refers to.
 
 As for `BasicJsonType`, with [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) these exceptions name the JSON
 Pointer path of the value within the document, and with
-[`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) its bytes in the source text (where
+[`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) its bytes in the source text (when
 [`materialize()`](materialize.md) sets them).
 
 ## Complexity