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>

lib/kernel/src/log.dart

@@ -62,3 +62,32 @@ void stderrSink(LogRecord r) {
   stderr.writeln(r);
   if (r.stackTrace != null) stderr.writeln(r.stackTrace);
 }
+
+/// Parse a level name (case-insensitive, trimmed) to a [LogLevel], or null if
+/// it is absent/blank/unknown — so an invalid source falls through to the next
+/// one in [resolveLogLevel] rather than crashing the boot.
+LogLevel? parseLogLevel(String? name) {
+  if (name == null) return null;
+  final n = name.trim().toLowerCase();
+  if (n.isEmpty) return null;
+  for (final l in LogLevel.values) {
+    if (l.name == n) return l;
+  }
+  return null;
+}
+
+/// Resolve the effective [Logger.minLevel] at boot — the dev/prod verbosity
+/// toggle (T-425). Highest precedence first:
+///
+///   1. `--dart-define=CLIDE_LOG=<level>` (baked into the build)
+///   2. the `CLIDE_LOG` environment variable
+///   3. the `app.log.level` setting
+///   4. a build-mode default: `warn` in release (a shipped app stays quiet),
+///      `info` in debug.
+///
+/// Each named source is parsed leniently; an unknown name is skipped, not
+/// fatal. The build-mode flag is passed in (rather than read here) to keep
+/// this Flutter-free — `main.dart` supplies `kReleaseMode`.
+LogLevel resolveLogLevel({required bool isRelease, String? dartDefine, String? envVar, String? settingValue}) {
+  return parseLogLevel(dartDefine) ?? parseLogLevel(envVar) ?? parseLogLevel(settingValue) ?? (isRelease ? LogLevel.warn : LogLevel.info);
+}