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/src/rust/api.dart

@@ -9,7 +9,7 @@ import 'package:flutter_rust_bridge/flutter_rust_bridge_for_generated.dart';
 import 'package:freezed_annotation/freezed_annotation.dart' hide protected;
 part 'api.freezed.dart';
 
-// These functions are ignored because they are not marked as `pub`: `runtime`, `session`
+// These functions are ignored because they are not marked as `pub`: `log_sink`, `runtime`, `session`
 // These function are ignored because they are on traits that is not defined in current crate (put an empty `#[frb]` on it to unignore): `clone`, `clone`, `clone`, `clone`, `clone`, `clone`, `fmt`, `fmt`, `fmt`, `fmt`, `fmt`, `fmt`, `from`, `from`, `from`
 
 /// Connect to a TeamSpeak-compatible server and return the initial
@@ -37,6 +37,12 @@ Future<void> startAudio() => RustLib.instance.api.crateApiStartAudio();
 Future<void> setPtt({required bool active}) =>
     RustLib.instance.api.crateApiSetPtt(active: active);
 
+/// User-initiated diagnostic export. Returns a multi-line text
+/// blob, redacted per the production policy, that the user can
+/// share or copy. DEC-016 forbids automatic uploads — this is the
+/// only path that surfaces logs.
+String exportDiagnostics() => RustLib.instance.api.crateApiExportDiagnostics();
+
 /// Wire the identity persistence store to a platform-private
 /// directory. Should be called once on app start after Flutter has
 /// resolved `getApplicationSupportDirectory()` (or equivalent).