# fix(read): prevent panic on negative slice index in a key path

## Problem

Looking up a key whose path contains a negative array index panics instead of
reporting "not found":

```go
c.LoadData(map[string]any{"arr": []any{"a", "b", "c"}})

c.GetValue("arr.-1") // panic: runtime error: index out of range [-1]
c.Exists("arr.-1")   // panic: runtime error: index out of range [-1]
```

Key paths frequently come from user input or configuration, so a stray
negative index crashes the caller.

## Root cause

`read.go`, in both `GetValue` and `Exists`, the slice cases parse the path
segment with `strconv.Atoi` and validate only the upper bound:

```go
case []any: // is array(load from file)
	i, err := strconv.Atoi(k)
	if err != nil || len(typeData) <= i { // no lower-bound check
		...
		return
	}
	item = typeData[i] // typeData[-1] panics
```

`strconv.Atoi("-1")` succeeds with `i == -1`, which passes `len(typeData) <= i`
(`3 <= -1` is false), so `typeData[-1]` is evaluated and panics. This affects
the `[]int`, `[]string` and `[]any` cases in both functions (six sites).

## Fix

Add a lower-bound check (`i < 0`) to every slice-index guard so a negative
index is treated as not found:

```go
if err != nil || i < 0 || len(typeData) <= i {   // GetValue
if err != nil || i < 0 || i >= len(typeData) {    // Exists
```

## Test

Added `TestConfig_GetValue_negativeIndex` in `read_test.go`, asserting that both
`GetValue("arr.-1")` and `Exists("arr.-1")` do not panic and report not found.

- Gate: `go test ./...` -> ok (PASS, exit 0, all packages)
- `go vet ./...` -> exit 0
- `go build ./...` -> exit 0
- `gofmt -l read.go read_test.go` -> clean

Counterfactual: reverting `read.go` makes the test fail with
`Panic value: runtime.boundsError{x:-1, y:3, ...}`.