docs: correct public contract edges

[jmgilman]

Summary

• separate duplicate-keyword parser failures from native argument-binding failures
• define inclusive final-value depth, exact Describe lookup, raw search-byte budgeting, and nil Execute context classification
• install both tutorial dependencies before presenting the first-server program
• add focused behavior regressions for duplicate keywords and value-depth boundaries

Testing

• mise exec -- go test ./internal/execution -run '^TestExecuteRejectsDuplicateKeywordSyntaxAsInvalidProgram$' -count=1
• mise exec -- go test ./internal/binding -run '^TestConvertFinalEnforcesDepthAndEncodedSize$' -count=1
• clean external Go 1.26.6 module: tutorial go get command, pre-tidy go build ., go mod tidy, and go run . using the unmodified fenced program
• mise exec -- moon run root:check