Use realistic Signal PDU payload examples

[RejectKid]

What changed

• replaces the hexadecimal Signal PDU payload in the primary README example with encoded audio supplied by an application component
• shows received Signal data being handed to an application decoder
• explains why Signal payload interpretation remains application-specific
• keeps non-byte-aligned payloads documented as an advanced wire-level case

Why

Users normally receive Signal bytes from the network or obtain them from an audio codec, tactical-data-link implementation, or file. The previous literal-byte example made ordinary library usage look lower-level than it is.

Validation

• git diff --check
• documentation-only change; no runtime behavior changed