YAML support for glz::generic

[stephenberry]

Fix YAML parsing with glz::generic

Fixes #2286

Summary

YAML parsing now works with glz::generic types. Previously, attempting to parse YAML into a glz::generic would fail with a template instantiation error because the YAML variant parser lacked the type detection logic present in the JSON parser.

Changes

YAML Variant Parser Enhancement (yaml/read.hpp)

Added sophisticated type detection for YAML variants, similar to JSON's approach:

• Type detection via syntax: The parser now examines the first character to determine the appropriate variant type:

  • { → object (flow mapping)
  • [ → array (flow sequence)
  • " or ' → string
  • t/T, f/F → boolean (true/false)
  • n/N, ~ → null
  • Digits, -, +, . → number

• New type traits: Added yaml_variant_* concepts and traits for variant type classification

• New from<YAML, std::nullptr_t> handler: Parses YAML null values (null, Null, NULL, ~)

• Fixed glaze_value_t handler: Now uses std::decay_t to properly strip reference qualifiers, matching the JSON implementation

Lookup Tables (yaml/common.hpp)

Added 256-byte lookup tables for efficient character classification:

• plain_scalar_end_table - Characters that terminate plain scalars
• whitespace_or_line_end_table - Whitespace and line endings
• line_end_or_comment_table - Line endings and comment marker
• flow_context_end_table - Flow context terminators

These replace long character comparison chains throughout the YAML parsing code for better performance.

[packit-as-a-service]

One of the tests failed for 772a864. @admin check logs None, packit dashboard https://dashboard.packit.dev/jobs/copr/3207702 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/10083085/