D5: Learn chapter — P4 reproducibility

[hhimanshu]

Chapter: reproducibility; volatile functions (NOW, RAND) and RecalcContext; why same context ⇒ identical bytes.
Acceptance: merged + CI-green. Gate G4 deliverable.

See truecalc/distributions plan §12 (learn-as-we-build, Fumadocs) and the chapter ladder. Tutorial-writer persona: explain to someone who has never opened a spreadsheet. Never assert spreadsheet behavior without a fixture ID from truecalc/core.