encode: bound nesting depth to prevent stack overflow on deep input

[martinholovsky]

The decoder gained a maxNestingDepth guard to fix a stack-exhaustion issue (GHSA-5wfc-hjrc-gq87 / GO-2026-5157), but the encoder was left unguarded.

The encoder’s only depth-related logic is cycle detection, which triggers solely on a repeated pointer:

// encode.go
case reflect.Ptr, reflect.Slice, reflect.Map:
    if e.pDepth++; e.pDepth > depthLimit {
        // ... errors only if value.Pointer() was seen before (a cycle)
    }

A deeply nested, non-cyclic value has a distinct pointer at every level, so the cycle check never fires and hjson.Marshal recurses until the goroutine stack overflows — a fatal, unrecoverable crash (a Go stack overflow cannot be recover()-ed).

Reproduce (still applies to v4.6.0):

var v any = "x"
for i := 0; i < 3_000_000; i++ { v = []any{v} }
_, _ = hjson.Marshal(v) // fatal error: stack overflow  (frames in encode.go)

Fix: reuse the existing pDepth counter to also enforce maxNestingDepth on the encode side, returning the same "Exceeded max depth (%d)" error the decoder already returns. No behavior change for normal input (nothing legitimately nests 10000 deep); the cycle check still catches true cycles early. Added a regression test (TestEncodeMaxDepth). go test ./... passes; gofmt clean.