feat: add native Rust logging facade

[tisonkun]

Application code currently needs to construct records manually or go through log to emit Logforth events. This adds an explicit-logger native facade over the existing record, value, and dispatch APIs.

let logger = logforth::builder()
    .dispatch(|d| d.append(logforth::append::Stderr::default()))
    .build();
logforth::info!(logger, {
    "completed" => 3u64,
    "healthy" => true,
}, "batch complete");
logforth::info!(logger, { "queue.depth" => 0 });
logger.flush();

• log! accepts every native level; trace! through fatal! provide the same field-map and message syntax. Macros borrow the logger and fields, evaluate expressions once, and prefilter before evaluating event data. Full-record filtering still runs; formatters may consume the same event more than once.
• Fields use standard From conversions into the existing kv::Value, with no new conversion trait. Scalars preserve their value kinds, strings and existing values are borrowed, and Value::display/debug, Value::list/map, and Value::none cover explicit text, nested data, and absence.
• The target is the calling module; source module/file/line/column identify the actual call site. Existing module filters apply. Custom targets remain available through RecordBuilder and the log bridge.
• Logger ownership and dispatch behavior stay unchanged. Build separate loggers for separate output policies, or share a logger with Arc<Logger>. The optional bridge can use the same logger. Flush stays explicit; fatal! does not terminate or flush.

This PR deliberately stops at native event construction. It adds no logger name, logger-bound context, field merging precedence, or context propagation model. Context governance needs a separate design covering event fields, diagnostics, collisions, and capture timing across appenders. Native Serde capture and serializer relocation are also deferred; the existing bridge serializer is unchanged.

The README and crate docs use the native facade as the primary entry point, and CI runs a featureless native example. Integration tests cover lazy evaluation, borrowed and nested fields, source metadata, full-record filtering, module filtering, independent logger policies, and native/bridge interoperability.

Validation: workspace tests with all features; cargo x lint; Rust 1.91 core and featureless facade tests; native integration tests with bridge-log and bridge-log-serde separately; the featureless native example; strict core/facade rustdoc. The CI matrix checks Linux, macOS, and Windows on stable and Rust 1.91.

Related to #205.