json: fix escapeIndex() on big-endian (s390x)

[dmkif]

Fixes #170 (full story there — chasing a Portainer bug on s390x/zCX, containers with OCI labels not listing).

Putting up a PR since I already had the change sitting locally, but genuinely not sure if this is the "right" fix from a project-conventions standpoint — happy to rework it if you'd rather do it differently. I'm still fairly new to Go so go easy on me if I got a convention wrong somewhere.

What I changed

escapeIndex() was casting 8-byte chunks of the string into a uint64 via unsafe.Pointer (the stringToUint64 helper), which reads native-endian. On big-endian (s390x) that's the wrong byte order for how bits.TrailingZeros64(...)/8 figures out the byte index afterwards — it returns the mirrored position (7-k instead of k) within the chunk, so it can point at the wrong byte and skip a character that should've been escaped.

I swapped that for binary.LittleEndian.Uint64 on each 8-byte block (same as parse.go already does elsewhere in this repo), and added the block offset i to the TrailingZeros64(...)/8 result so the index comes out right no matter the host endianness.

Since stringToUint64 isn't used anywhere else after that, I removed it and the now-unused unsafe import from json/string.go. Left sliceHeader alone since it's still used in json/codec.go.

Testing

Added TestEscapeIndexBigEndian to json/string_test.go:

• runs escapeIndex against a plain byte-by-byte reference scanner for a handful of strings with the special character at different positions (including right around the 8-byte chunk boundary, since that's where things went wrong for me)
• also round-trips each string through this package's Marshal and stdlib encoding/json's Unmarshal to check the actual JSON output is valid and comes back as the same string, similar to the reproducer in escapeIndex() breaks string escaping on big-endian (s390x) #170

On amd64:

• go build ./... — fine
• go vet ./json — some pre-existing warnings in other test files, unrelated to this change, didn't touch those
• go test ./json -run TestEscapeIndexBigEndian — passes

I don't have a Go toolchain on the s390x box itself (zCX/Tumbleweed, not really meant for dev work), so I built and ran the test on a big-endian setup I have access to elsewhere — it failed against the old code and passes with this change.

Only touched json/string.go and json/string_test.go, didn't touch go.mod/go.sum.