feat: report POC and slice QP per picture in mp2ts-nallister

[tobbee]

Summary

• Parse the first slice header of each picture for both AVC and HEVC to expose poc (pic_order_cnt_lsb) and qp (initial SliceQPY = 26 + pic_init_qp_minus26 + slice_qp_delta) alongside the existing imgType in mp2ts-nallister JSON output.
• Falls back to the lightweight slice-type parser when SPS/PPS aren't available yet.
• Golden files updated for bbb_1s (AVC) and obs_hevc_aac (HEVC) test cases.

Test plan

• go test ./...
• go run ./cmd/mp2ts-nallister -max 5 internal/testdata/bbb_1s.ts shows "poc" and "qp" per frame
• go run ./cmd/mp2ts-nallister -max 5 internal/testdata/obs_hevc_aac.ts shows "poc" and "qp" per frame