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

GetValue and Exists validated only the upper bound of a numeric key-path
segment, so a negative index such as "arr.-1" passed the check and indexed
the slice with -1, causing an index-out-of-range panic. Add a lower-bound
check (i < 0) to the []int, []string and []any cases in both functions.

PR.md

@@ -0,0 +1,58 @@
+# 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, ...}`.