Part 4 of the logging stack (builds on #724). Adds the
application-facing macros — the part most callers actually use.

**What's here**
- `ICEBERG_LOG_{TRACE,DEBUG,INFO,WARN,ERROR,CRITICAL,FATAL}`, the
generic `ICEBERG_LOG(level, …)`, `ICEBERG_LOG_TO(logger, …)`, and
`ICEBERG_LOG_RUNTIME_FMT` for runtime format strings.
- `ICEBERG_LOG_ACTIVE_LEVEL` is a compile-time floor: statements below
it are removed entirely. `ICEBERG_LOG_FATAL` always emits, flushes, then
aborts.
- Filtering is decided only by `ShouldLog()`; formatting happens only on
the taken path and never throws (a bad format falls back safely).
- Generic/runtime-format macros use the same lock-free fast path as the
fixed-severity ones (the slot lock is taken only on the fatal/abort
path).
- Bare `LOG_*` aliases are opt-in via `ICEBERG_LOG_SHORT_MACROS`.

**Build note** — sets `/Zc:preprocessor` on MSVC, required for the
`__VA_OPT__` used by the macros.

**Examples** — every macro takes a `std::format` string + args. The
rendered line depends on the active backend; with the default
`CerrLogger` it looks like:

```
ICEBERG_LOG_TRACE("entering scan for {}", table);
  2026-06-16T10:59:41.186Z trace [12345] table_scan.cc:88] entering scan for db.t
ICEBERG_LOG_DEBUG("cache miss key={}", key);
  2026-06-16T10:59:41.186Z debug [12345] cache.cc:42] cache miss key=manifest-7
ICEBERG_LOG_INFO("loaded {} manifests in {} ms", n, ms);
  2026-06-16T10:59:41.186Z info [12345] table_scan.cc:91] loaded 5 manifests in 12 ms
ICEBERG_LOG_WARN("retry {} after {}", attempt, err);
  2026-06-16T10:59:41.186Z warn [12345] io.cc:51] retry 2 after timeout
ICEBERG_LOG_ERROR("commit failed: {}", status);
  2026-06-16T10:59:41.186Z error [12345] txn.cc:77] commit failed: conflict
ICEBERG_LOG_CRITICAL("metadata unreadable at {}", path);
  2026-06-16T10:59:41.186Z critical [12345] meta.cc:30] metadata unreadable at s3://b/m.json
ICEBERG_LOG_FATAL("unrecoverable: {}", reason);   // emits, flushes, then std::abort()
  2026-06-16T10:59:41.186Z fatal [12345] boot.cc:19] unrecoverable: bad config
```

Less common forms: `ICEBERG_LOG(level, …)` (runtime severity),
`ICEBERG_LOG_TO(logger, level, …)` (explicit logger),
`ICEBERG_LOG_RUNTIME_FMT(level, fmt_string, args…)` (non-literal
format). The same examples are documented inline in `logger.h`.

**Tests** — `macros_test` / `macros_active_level_test`: formatting,
level gating, "argument not evaluated when disabled", compile-time
stripping, never-throws, and FATAL-aborts death tests. clang/libc++.

This pull request and its description were written by Isaac.