Zen-style trace log events (#1006)

Replaces the old (not fully implemented) UE `Logging.*` sink with a typed `ZenLog.*` trace path that preserves structured fmt args end-to-end, so the zen trace analyzer (and future consumers) can re-render log messages with full formatter support.

- Hook `Logger::Log` to tap `fmt::format_args` before `vformat` renders them, and emit three new events on a dedicated `ZenLogChannel`: `Category`, `MessageSpec`, `Message`. Args are serialized as `[count][descriptors][payload]` with distinct categories for bool, int, float, and string. Custom formatters fall back to a pre-rendered string.
- Bool has its own wire category so `{}` renders as `true`/`false` and `{:d}` as `1`/`0`.
- Zen `LogLevel` is translated to UE `ELogVerbosity` on emit so severity filtering works consistently.
- Extend the zen trace analyzer to decode `ZenLog.*` via `fmt::vformat` + `dynamic_format_arg_store` — nested widths, chrono specs, etc. all work. Strings are passed as views directly from the event payload (which outlives the format call) rather than copied through a pool.
- Retire the old `TraceSink` stub; the typed path supersedes it.
- Switch `--trace=default` alias from `cpu,log` to `cpu,zenlog`.
- Add `__int128` overloads to the arg encoder guarded by `FMT_USE_INT128` so fmt's int128 dispatch resolves unambiguously on clang/gcc. MSVC and clang-cl are unaffected.

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