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

[dualfroz]

Problem

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

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:

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:

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, ...}.