Pin verbatim export of snippet and Blueprint source

[sirreal]

Settles a policy question left open during the #262 rebase: after the targeted redesign of global-name prefix stripping, doc.code_snippets[].code / expected_output and setup_blueprints receive no stripping at all — master's blanket array_walk_recursive used to touch them. That is the correct behavior — snippets and Blueprint definitions are executable, verbatim source, not prose — but nobody had stated it or pinned it.

This adds a fixture whose snippet code, expected output, and Blueprint writeFile data all contain \-qualified global names, and asserts they export exactly as authored.

The test passes on fix-254. The same test run against master fails: the blanket walk rewrites the snippet to echo Docs_Global::greet(); and the expected output to Hello from Docs_Global — demonstrating that the pin captures a real difference in #262's behavior, not a tautology. (Master happens to leave this particular Blueprint data alone because its backslashes follow a newline and a quote rather than a space — which only underlines how incidental the old stripping was.)

Targets fix-254 to land with #262.

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