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.

[Voyagerroc-Lab]

Reviewed 4c25f58 against the documented map[string]interface{} / []interface{} selector contract. Indexed writes now update the final slice element without replacing the containing map value. The regression assertions check the underlying slice directly, avoiding the old false positive where Get("names[0]") returned the replacement scalar. The nested-index, nil-assignment, out-of-range, and non-slice-read cases are covered as well.

Validation on Windows / Go 1.27.0: 257 tests passed with go test ./..., and go vet ./... passed. Local go test -race ./... could not run because CGO is disabled and no C compiler is available; the PR's six Go 1.20–1.25 CI jobs are successful and use the repository's race-enabled Taskfile test command.

Review and validation performed with OpenAI Codex at the account owner's request.