feat: add std::span support for C++20 builds

[k-wasniowski]

Add std::span support for C++20 builds

Summary

Uses std::span for the public byte-view types when building with C++20 and retains gsl_lite::span as the fallback for older language standards.

This backports the span compatibility layer from the main branch to the v1 branch.

Changes

• Added include/sframe/span.h to select the available span implementation:
  • std::span when supported by the standard library.
  • gsl_lite::span for pre-C++20 builds.
• Updated include/sframe/sframe.h to use the compatibility span alias for input_bytes and output_bytes.
• Removed the direct public API dependency on the gsl namespace alias.

Compatibility

• Existing C++11 builds continue to use gsl_lite::span.
• C++20 consumers use std::span without API changes to input_bytes or output_bytes.
• No wire-format or runtime behavior changes.

Validation

• Built the sframe target using the project's configured C++11 mode.
• Compiled a C++20 translation unit asserting that input_bytes and output_bytes resolve to the corresponding std::span types.
• Verified the patch with git diff --check.

[Copilot]

Copilot review overview

🟡 Changes recommended

Add <stdexcept> directly and test a C++20 consumer build.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 2

Open (2)

• Missing direct <stdexcept> include in public header · New
• Missing C++20 consumer build and link test · New

What changed in this PR

Adds C++20 std::span support while retaining gsl_lite::span for older standards.

Changes:

• Adds conditional span selection.
• Updates public byte-view aliases.
• Removes the direct public gsl namespace dependency.

File	Summary
include/​sframe/​span.h	Selects std::span or gsl_lite::span; C++20 consumer coverage is missing.
include/​sframe/​sframe.h	Uses compatibility span aliases; needs a direct <stdexcept> include.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Unresolved ABI, preprocessing compatibility, and header self-containment issues block approval.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 4

Open (4)

• Inconsistent span ABI between library and C++20 consumers · New
• CMake standard mismatch causes span signature link failures · New
• Missing C++20 consumer build and link test
• Missing direct <stdexcept> include in public header