docs(AFE): debugger FAQ for REGEX gaps + JSON_GET example

[FireZombieCoder]

Summary

• Document that AFE’s formula debugger is a partial evaluator separate from Excel calc, including REGEX / unimplemented-function / spilled-array cases (links to Add REGEX support #45, Debugging expressions using ANCHORARRAY #41, and AFE #83).
• Note underscore naming for module functions vs dotted path arguments.
• Add examples/JSON_GET.md — small JSON_* LAMBDA module using REGEXEXTRACT, with an explicit debugger caveat.

Test plan

• FAQ renders correctly on GitHub
• Example code pastes into an AFE module without syntax errors (underscores, trailing ;)
• Grid: JSON_GET on flat JSON works when REGEX is available in Excel
• Debugger: confirm REGEX still unsupported until Add REGEX support #45 lands (expected)

Closes nothing — supports discussion on #45.

[FireZombieCoder]

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