Lowercase all mdsnippets snippet names

[jeremydmiller]

Summary

Closes #2530.

Why

The latest MarkdownSnippets.Tool (28.2.0) is case-sensitive when matching <!-- snippet: NAME --> references against #region NAME declarations. Older versions (e.g. 24.5.1) were case-insensitive — but contributors installing the current release would hit build failures on any mixed-case mismatch.

Verified empirically with both versions:

• mdsnippets 24.5.1: <!-- snippet: SAMPLE_X --> against #region sample_x → matches ✓
• mdsnippets 28.2.0: same pair → fails with Failed: Missing snippets: SAMPLE_X ✗

This PR forces every snippet name to lowercase so the docs build cleanly on any version.

What changed

• All <!-- snippet: NAME --> references in *.md lowercased
• All #region NAME declarations that follow snippet naming conventions (sample_* / snippet_* prefix) lowercased
• Code-organization regions with multi-word names (e.g. #region Designer generated code, #region Test setup) left untouched
• C# changes are preprocessor-directive renames only — zero behavioral impact
• Markdown diff is large because mdsnippets regenerated the inner anchor IDs with the new lowercase names

499 files changed (mostly markdown).

Verification

$ mdsnippets   # version 28.2.0
...
MarkdownProcessor Finished. 35ms
Finished 222ms

Clean run, no missing-snippet failures. Also verified the existing 24.5.1 install still works.

🤖 Generated with Claude Code