NonSharedUint8Array type now exported; user_data frame metadata added

Patch Changes

• Conform placeholder sections to the canonical fmtp for each shared payload - #1993 (@lukasIO)

• Include the NonSharedUint8Array type polyfill in the published declarations. - #1997 (@1egoman)

  The type was declared in an ambient .d.ts that was resolved during our own build but never emitted to dist, so the published .d.ts files referenced a type consumers could not resolve — surfacing as Cannot find name 'NonSharedUint8Array' under skipLibCheck: false and as Unable to follow symbol for "NonSharedUint8Array" in API Extractor (which broke downstream packages such as @livekit/components-*). The polyfill is now an exported type that is imported explicitly wherever it is used, so it ships in dist and the published types type-check standalone.

• Add support for user_data frame metadata trailer type - #1983 (@chenosaurus)