Add per-picture SEI payload injection API

[kpchoi]

Adds a public API to attach SEI payloads (HDR10 mastering display / content
light level, HDR10+ and other ITU-T T.35 messages, etc.) to individual
pictures, so callers like the FFmpeg wrapper can pass HDR metadata through.

Usage: fill an XEVE_SEI list and set imgb->pdata[XEVE_IMGB_SEI_SLOT] /
imgb->ndata[XEVE_IMGB_SEI_SLOT] = XEVE_SEI_MAGIC before xeve_push(). The
payloads are deep-copied during the push, carried through frame reordering,
and written as prefix SEI NAL units (one per payload) into the access unit of
that picture. Payload bytes are passed through as-is; the library only does
the sei_message framing. The existing spare ndata/pdata fields are used, so
the ABI is unchanged; the internal SEI type/struct definitions move to the
public header.

Also includes a README pass: SEI usage in the programming guide, typo fixes
(libxexe -> libxeve), fps option now documented as accepting fractions, and a
note that ARM hosts no longer need the cross-compile flags.

Verified with an encode/decode round trip against the xevd counterpart
(mpeg5/xevd#94): 16 frames with per-frame T.35 + MDCV payloads and bframes=15
come back frame-accurate on the decoder side; streams without SEI are
bit-exact with master; MAIN/BASE and x86/aarch64 builds pass; decoder-side
AddressSanitizer run is clean.

[mpeg5]

LGTM