feat(diagnostics): A.3 — redacted in-memory log sink + user-initiated export

Replaces the diagnostics scaffold with the production redaction
policy + a user-initiated export path that satisfies DEC-016 (no
automatic uploads).

* `chanora_diagnostics::Redactor` applies the six policy rules to
  every captured log line: `$HOME` paths → `[home]`; IPv4 + IPv6
  literals → `[ip]`; email-shaped strings → `[email]`; long
  base64-ish tokens → `[token]`; substrings registered with
  `KnownSecretRegistry` → `[REDACTED]`. The registry implements
  SS-AUD-003 defence-in-depth: storage adapters can register
  secrets as they cross out of the keyring so an accidental
  `Debug` print is still scrubbed at write time.
* `InMemoryLogSink` is a bounded ring buffer (cap 500 lines in the
  bridge) that always passes lines through the redactor before
  storing them. `RedactingLogLayer` plugs it into `tracing-
  subscriber` alongside the existing logcat / fmt layers.
* `DiagnosticExport::from_sink` builds a plaintext blob — already
  redacted — combining free-form metadata (crate version, target
  os/arch) with the retained log tail. `bridge::api::
  export_diagnostics()` is the Flutter-facing entrypoint
  (`#[frb(sync)]`).
* `bridge_init` now installs the redaction layer on both Android
  and desktop hosts, switching from the global `fmt::init()`
  shortcut to a layered `Registry` so the in-memory sink can sit
  side-by-side with the platform sink.
* Flutter adds a bug-report icon to the AppBar; tapping it opens a
  scrollable monospace dialog with Copy and Close actions. New
  `diagnosticsAction` / `copyAction` / `closeAction` strings land
  in `app_en.arb` + `app_zh.arb`.

Tests cover the redaction matrix (IPv4, IPv6, email, long tokens,
known secret), the ring buffer capacity, and the full
`DiagnosticExport::to_text()` round-trip — 9/9 green.

Live-verified on Moto G: the dialog rendered a multi-line transcript
with `[ip]`, `[token]`, `[home]` substitutions, the metadata block
showed `target_os=android` `target_arch=aarch64`, and Copy placed
the same text on the clipboard.

apps/chanora_flutter/lib/l10n/app_en.arb

@@ -12,6 +12,9 @@
   "connectAction": "Connect",
   "disconnectAction": "Disconnect",
   "refreshAction": "Refresh",
+  "diagnosticsAction": "Diagnostics",
+  "copyAction": "Copy",
+  "closeAction": "Close",
   "startAudioAction": "Start audio",
   "pttHoldToTalk": "Hold to talk",
   "pttTransmitting": "Transmitting…",

apps/chanora_flutter/lib/l10n/app_zh.arb

@@ -11,6 +11,9 @@
   "connectAction": "连接",
   "disconnectAction": "断开连接",
   "refreshAction": "刷新",
+  "diagnosticsAction": "诊断信息",
+  "copyAction": "复制",
+  "closeAction": "关闭",
   "startAudioAction": "启动语音",
   "pttHoldToTalk": "按住说话",
   "pttTransmitting": "正在发送…",

apps/chanora_flutter/lib/l10n/generated/app_localizations.dart

@@ -139,6 +139,24 @@ abstract class AppL10n {
   /// **'Refresh'**
   String get refreshAction;
 
+  /// No description provided for @diagnosticsAction.
+  ///
+  /// In en, this message translates to:
+  /// **'Diagnostics'**
+  String get diagnosticsAction;
+
+  /// No description provided for @copyAction.
+  ///
+  /// In en, this message translates to:
+  /// **'Copy'**
+  String get copyAction;
+
+  /// No description provided for @closeAction.
+  ///
+  /// In en, this message translates to:
+  /// **'Close'**
+  String get closeAction;
+
   /// No description provided for @startAudioAction.
   ///
   /// In en, this message translates to:

apps/chanora_flutter/lib/l10n/generated/app_localizations_en.dart

@@ -30,6 +30,15 @@ class AppL10nEn extends AppL10n {
   @override
   String get refreshAction => 'Refresh';
 
+  @override
+  String get diagnosticsAction => 'Diagnostics';
+
+  @override
+  String get copyAction => 'Copy';
+
+  @override
+  String get closeAction => 'Close';
+
   @override
   String get startAudioAction => 'Start audio';
 

apps/chanora_flutter/lib/l10n/generated/app_localizations_zh.dart

@@ -29,6 +29,15 @@ class AppL10nZh extends AppL10n {
   @override
   String get refreshAction => '刷新';
 
+  @override
+  String get diagnosticsAction => '诊断信息';
+
+  @override
+  String get copyAction => '复制';
+
+  @override
+  String get closeAction => '关闭';
+
   @override
   String get startAudioAction => '启动语音';