tp+ui: render (is_pid)/(is_tid) args as process/thread references

Args produced from an (is_pid)/(is_tid) proto field annotation carried
their upid/utid into trace_processor but were stored as a plain integer,
so the UI could not tell them apart from any other int and rendered them
as a bare number.

Give the value its own identity end to end, mirroring how kPointer already
distinguishes a pointer from an int:

  - Add Variadic::kUpid / kUtid (value_type 'upid' / 'utid'), so the
    semantic survives into the args table and is queryable in SQL.
  - AddUpid/AddUtid store the new types instead of Variadic::Integer.
  - arg_set_to_json serialises them as a typed reference object,
    {"__ref": "process"|"thread", "id": <n>}, keyed by the target table.
  - The args details tree keeps the upid/utid arg as the raw id and adds a
    companion row, keyed by rewriting the id suffix (caller_upid ->
    caller_process_name, x_utid -> x_thread_name), showing the process/thread
    name -- without the pid/tid -- resolved live from the process/thread
    table so it never goes stale. If the key lacks the expected suffix the
    name row is skipped.

Everywhere else the new types behave exactly like an integer (they are
one), so the remaining Variadic switches fall through to the kInt path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Change-Id: I8765552630abf6396988c0a53987d27db1b1d2a1