Preserve slices when setting an indexed value

[vitalivo]

Summary

Set("names[0]", "Mat") replaces the entire names slice with "Mat". The existing test misses the data loss because Get("names[0]") also ignores an index on a non-slice and returns that scalar.

For indexed writes into []interface{}, traverse the indices and update the final element instead of assigning the map key. Return a missing value when a read attempts to index a non-slice. Regression tests check that other elements survive, nested writes work, nil can be assigned, and out-of-range writes preserve the slice. Both new tests fail before the fix.

Checklist

• Tests pass: ran the Taskfile's go test -race ./... with coverage enabled (99.1%).
• Code style is correct: ran the Taskfile's gofmt -d -s *.go and go vet ./... commands; both are clean.

Validated with Go 1.27.1.