[BUG]: This PR addresses recurring C4819 compiler warnings on Windows when using pybind11 with non-Unicode code pages (e.g., CP936):

[bienanguo]

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What version (or hash if on master) of pybind11 are you using?

pybind11-3.0.1

Problem description

Replace non-ASCII characters in comments with ASCII equivalents to fix C4819 warnings on Windows.
This PR addresses recurring C4819 compiler warnings on Windows when using pybind11 with non-Unicode code pages (e.g., CP936):

warning C4819: The file contains a character that cannot be represented in the current code page (936). Save the file in Unicode format to prevent data loss.

The warnings are triggered by a few non-ASCII Unicode characters (e.g., en-dash –, em-dash —, smart quotes, or mathematical symbols) present in source comments. While harmless, these warnings clutter build logs—especially in large projects or CI pipelines—and may confuse new users.

This change:

Replaces all non-ASCII characters in comments only with standard ASCII equivalents (e.g., – → -, “/” → ", ‘/’ → ').
Does not modify any code, identifiers, string literals, or functionality.
Maintains comment clarity and readability.
Improves compatibility with legacy Windows toolchains without requiring file encoding changes.
Files affected:

include/pybind11/cast.h
include/pybind11/detail/struct_smart_holder.h
(and any other files found to contain non-ASCII comment characters)
This is a purely cosmetic and compatibility-focused change with zero runtime impact.

Fixes: (optional: reference an issue if one exists, e.g., #XXXX)

Reproducible example code

Is this a regression? Put the last known working version here if it is.

Not a regression