feat(finalmask): header-custom variables, sudoku UDP mask and noise reset ranges

xray-core 26.6 grew the finalmask surface the editor could not express,
and the finalmask watch drops any key without a registered field, so a
config written by hand lost these keys on the next save:

- header-custom items can capture their bytes into a variable, reuse one
  or emit a transform expression; a Kind select keeps each item to exactly
  one kind, as the core requires, and only parsed JSON reaches transform;
- TCP header-custom gains the errors group (the reply sent when a client
  group does not match), UDP header-custom its prefix/standalone mode;
- sudoku is offered as a UDP mask, and share links keep it in fm=;
- the noise reset is an Int32Range, so it now takes "60-120" as well.

The new golden fixture is saved unchanged by the outbound editor and
built by xray-core in TestGoldenStreamFixturesBuildInXray.

frontend/src/schemas/protocols/stream/finalmask.ts

@@ -20,7 +20,7 @@ export const TcpMaskSchema = z.object({
 export type TcpMask = z.infer<typeof TcpMaskSchema>;
 
 // 'udphop' is client-only in xray-core (it refuses to wrap a server socket),
-// so it round-trips here but is deliberately absent from the mask dropdown.
+// so only the client-side mask editors offer it.
 export const UdpMaskTypeSchema = z.enum([
   'salamander',
   'mkcp-legacy',