Fourth block: the application-facing macros, the only part most callers touch.

- ICEBERG_LOG_{TRACE,DEBUG,INFO,WARN,ERROR,CRITICAL,FATAL} plus the generic
  ICEBERG_LOG(level, ...), ICEBERG_LOG_TO(logger, level, ...) for an explicit
  logger, and ICEBERG_LOG_RUNTIME_FMT for a runtime (non-literal) format string.
- ICEBERG_LOG_ACTIVE_LEVEL is a compile-time severity floor: statements below it
  are removed entirely via `if constexpr` (no format call site, no source
  location emitted). ICEBERG_LOG_FATAL is never gated by the floor -- its abort
  is always compiled in; it emits, best-effort Flush()es the same logger it
  emitted to, then std::abort().
- Filtering is decided solely by Logger::ShouldLog(); formatting is wrapped in
  try/catch so logging never throws (a format failure routes to EmitFormatError).
- Bare Java-style aliases (LOG_INFO, ...) are opt-in via ICEBERG_LOG_SHORT_MACROS
  to avoid polluting consumers / colliding with glog/abseil.

Header-only addition to logger.h. macros_test covers injection, the
guard-before-format short-circuit, never-throws, and FATAL aborts;
macros_active_level_test verifies compile-time stripping in a kOff translation unit.

Co-authored-by: Isaac

Note in FormatAndEmit and the ICEBERG_INTERNAL_LOG comment that the only throws
are std::format_error/std::bad_alloc (same recovery), and that catching
std::exception rather than (...) lets a non-std unwind such as
abi::__forced_unwind (thread cancellation) propagate.

Co-authored-by: Isaac

Adopt the structure from the apache#824 prototype (keeping our if constexpr gating):
- logger.h is now the C++ API only; ICEBERG_LOG_* macros move to log_macros.h,
  and the bare LOG_* aliases move behind an opt-in short_log_macros.h (replacing
  the ICEBERG_LOG_SHORT_MACROS define).
- Dedup the five macro bodies onto shared internal helpers (EmitIfEnabled +
  LogToCurrent / LogToCurrentRuntime / LogToExplicitRuntime / LogFatal) that take
  a lazy [&]()->std::string message thunk invoked only past ShouldLog, so disabled
  logs still don't evaluate their args. Keeps catch(...), the GetCurrentLogger()
  fatal path, and if constexpr compile-time floor.
- VFormat moves to log_macros.h (only ICEBERG_LOG_RUNTIME_FMT uses it).

Co-authored-by: Isaac

…review ⑤)

Address the extensibility comment: let an embedder (JNI, Python host, crash
reporter) run a hook on the fatal path before std::abort() to flush resources or
print a stack trace.

- Add FatalHandler = std::function<void(const std::source_location&,
  std::string_view)> plus thread-safe SetFatalHandler/GetFatalHandler (leaked,
  teardown-safe slot in logger.cc).
- Wire it into LogFatal: format the message once, emit-if-enabled + flush, run the
  handler (message passed even when the record is filtered out), then abort. A
  throwing handler cannot prevent the abort.
- Death tests: handler runs with the formatted message, receives the call-site
  location, and still fires when the record is suppressed.

Co-authored-by: Isaac