SUKU File Explorer panel initial implementation

[SukuWc]

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Visit the preview URL for this PR (updated for commit 938fc04):

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[SukuWc]

Firmware (grid-fw):

• FETCH handler now sends NACKNOWLEDGE for all recall_configuration failure
  cases (inactive page, nvm busy, invalid element) instead of silently
  dropping the request

Editor (grid-editor):

• NACKNOWLEDGE added to InstructionClass enum
• validate_incoming now detects NACK by checking class_instr first, matches
  on BRC + class_name + LASTHEADER, and calls waiter.destroy() to terminate
  the request as an error (no more infinite retry)
• FetchConfig filter gets LASTHEADER: null for proper NACK correlation