feat(log): crash-survivable FileLogSink + dev/prod verbosity toggle (T-432)

First increment of the observability epic (T-425), the productive pivot after
the ConPTY freeze refused to reproduce on CI: if we can't reproduce it, make
the next occurrence leave evidence.

- FileLogSink (lib/kernel/src/file_log_sink.dart): synchronous, crash-survivable
  LogSink. Appends each record as one JSON line to a size-rotated file; fsyncs
  warn/error + risky-source (pty/ffi/conpty/watchdog) records immediately so the
  last breadcrumb is on disk before a hard death, batches the rest on a timer.
  Never throws. Flutter-free → unit-tested under dart test against a temp dir.
- logDirectory() (paths.dart): persistent per-platform log dir (LOCALAPPDATA /
  ~/Library/Logs / $XDG_STATE_HOME) — durable across reboot, unlike the
  ephemeral socketDirectory.
- resolveLogLevel() (log.dart): the requested dev/prod toggle. CLIDE_LOG
  dart-define → CLIDE_LOG env → app.log.level setting → warn(release)/info(debug).
  Lenient parse; an invalid source falls through.
- Boot wiring (facade.boot + main.dart): FileLogSink leads the sink chain (so a
  crash records before the volatile stderr/ring sinks) and the resolved level
  sets Logger.minLevel.

Tests: FileLogSink (JSON shape, error/stack, rotation cap, append-across-restart,
timer-cancel), resolveLogLevel precedence + fall-through, logDirectory per-OS.
Coverage gate 95.11%.

Follow-ups under T-425: live toggle CLI/command/chip (T-433), FFI breadcrumbs
(T-434), watchdog isolate (T-435), CI artifact wiring (T-436).

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

test/kernel/src/log_test.dart

@@ -75,4 +75,46 @@ void main() {
       expect(got, isEmpty);
     });
   });
+
+  group('parseLogLevel', () {
+    test('parses each level name case-insensitively, trimmed', () {
+      for (final l in LogLevel.values) {
+        expect(parseLogLevel(l.name), l);
+        expect(parseLogLevel(l.name.toUpperCase()), l);
+        expect(parseLogLevel('  ${l.name}  '), l);
+      }
+    });
+
+    test('null / blank / unknown → null', () {
+      expect(parseLogLevel(null), isNull);
+      expect(parseLogLevel(''), isNull);
+      expect(parseLogLevel('   '), isNull);
+      expect(parseLogLevel('verbose'), isNull);
+    });
+  });
+
+  group('resolveLogLevel (dev/prod verbosity toggle)', () {
+    test('build-mode default when no source is set: warn release / info debug', () {
+      expect(resolveLogLevel(isRelease: true), LogLevel.warn);
+      expect(resolveLogLevel(isRelease: false), LogLevel.info);
+    });
+
+    test('precedence: dartDefine > env > setting > default', () {
+      // setting only
+      expect(resolveLogLevel(isRelease: true, settingValue: 'debug'), LogLevel.debug);
+      // env beats setting
+      expect(resolveLogLevel(isRelease: true, envVar: 'error', settingValue: 'debug'), LogLevel.error);
+      // dartDefine beats both
+      expect(resolveLogLevel(isRelease: false, dartDefine: 'trace', envVar: 'error', settingValue: 'debug'), LogLevel.trace);
+    });
+
+    test('an unknown/blank higher source falls through to the next', () {
+      // empty dart-define (the String.fromEnvironment default) is skipped
+      expect(resolveLogLevel(isRelease: true, dartDefine: '', envVar: 'info'), LogLevel.info);
+      // garbage env falls through to the setting
+      expect(resolveLogLevel(isRelease: true, envVar: 'loud', settingValue: 'warn'), LogLevel.warn);
+      // all invalid → build-mode default
+      expect(resolveLogLevel(isRelease: false, dartDefine: 'x', envVar: 'y', settingValue: 'z'), LogLevel.info);
+    });
+  });
 }