Add an experimental basic_static_cstring template in example/

This introduces an alternative to basic_static_string designed for use
in POD types: Trivially copyable, having a sizeof == N + 1, with no
embedded NULs.

Placed in example/ to gather user feedback before committing to a public
API. See <#23>.

Author: gennaroprota

.github/workflows/example.yml

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+#
+# Copyright (c) 2025 Gennaro Prota (gennaro dot prota at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/boostorg/static_string
+#
+
+name: Example (basic_static_cstring)
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+      - develop
+      - bugfix/**
+      - feature/**
+      - fix/**
+      - github/**
+      - pr/**
+    paths-ignore:
+      - LICENSE
+      - meta/**
+      - README.md
+
+env:
+  B2_TARGETS: libs/$SELF/example
+
+jobs:
+  call-boost-ci:
+    name: Run Boost.CI
+    uses: boostorg/boost-ci/.github/workflows/reusable.yml@master
+    with:
+      exclude_cxxstd: '98,03,0x,11,14,17'
+      enable_pr_coverage: false
+      enable_multiarch: false

example/static_cstring/Jamfile

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+#
+# Copyright (c) 2025 Gennaro Prota (gennaro dot prota at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/boostorg/static_string
+#
+
+import testing ;
+
+project
+  : requirements
+    <include>../../include
+    <warnings>extra
+    <cxxstd>20
+  ;
+
+run static_cstring_test.cpp ;

example/static_cstring/README.md

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+This directory contains an experimental implementation of `basic_static_cstring`, which differs from `basic_static_string` in the following ways:
+
+|                     | `basic_static_cstring` | `basic_static_string` |
+|---------------------|------------------------|-----------------------|
+| Layout              | `sizeof == N + 1`      | Has size member       |
+| Embedded NULs       | Not supported          | Supported             |
+| Trivially copyable  | Yes                    | No                    |
+
+Additionally, when `N <= UCHAR_MAX`, `basic_static_cstring` employs an optimization that avoids calling `std::strlen()` to compute the size.
+
+This work stems from [boostorg/static_string#23](https://github.com/boostorg/static_string/issues/23).
+
+If you believe `basic_static_cstring` should become part of the public API, please share your feedback on the Boost mailing list.