feat(scaffold): create product workspace + Rust crates + Flutter app

Implements the canonical implementation directory layout adopted by
DEC-022 (register v0.9.5). Closes the scaffolding phase; no PoC code
has been promoted in yet (per proof-of-concept-plan.md §4 a PoC is
not product code unless explicitly promoted).

Rust workspace
==============

Top-level Cargo.toml declares seven workspace members:

  core/chanora_core           top-level Rust API + orchestration
  crates/chanora_protocol     tsclientlib isolation (SAD-067, SysDes-011/029)
  crates/chanora_state        state sync, reducers, deltas
  crates/chanora_audio        capture, DSP, Opus, jitter, mixer, playback
  crates/chanora_storage      non-secret DB + platform secure store
  crates/chanora_diagnostics  logs, redaction, export
  crates/chanora_bridge       typed Flutter/Rust DTOs

Workspace-wide pins:
  license      = "MIT OR Apache-2.0"   (DEC-020)
  rust-version = "1.95"
  edition      = "2021"

The Flutter app (apps/chanora_flutter) is NOT a Cargo workspace
member; it is owned by the Flutter / Gradle toolchain and is in the
workspace exclude array along with every poc/* spike.

Each crate ships:
  * a Cargo.toml referring to workspace.dependencies pins;
  * a lib.rs with #![forbid(unsafe_code)] + #![warn(missing_docs)],
    a typed Error enum, and the public types relevant to the
    subsystem's role per SAD §7.2;
  * minimal unit tests so
running 1 test
test tests::defaults_match_decisions ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 1 test
test tests::it_compiles ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 1 test
test tests::session_can_be_constructed ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 1 test
test tests::marker_matches_poc ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 1 test
test tests::it_compiles ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 1 test
test tests::state_transitions_compile ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 1 test
test tests::it_compiles ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s is non-empty.

chanora_core's CoreError type-wraps every subsystem error via
#[from] so callers can match on origin without parsing strings.

chanora_audio's AudioEffects struct defaults all four effects to
true, matching DEC-007 (AEC), DEC-008 (AGC), DEC-009 (NS),
DEC-010 (HPF). A unit test pins this so a future regression that
flips a default fails immediately.

chanora_diagnostics exports REDACTION_MARKER = "[REDACTED]",
identical to the PoC's marker so audit grep patterns survive the
promotion.

chanora_bridge's BridgeError is Serialize + Deserialize so it can
flow across the FRB 2.x boundary (DEC-014).

Empirical verification: cargo check --workspace clean, cargo test
--workspace clean (7 unit tests + 7 doc-test runners, all passing)
against Rust 1.95.0 stable.

Flutter app
===========

apps/chanora_flutter created with .

DEC-004 applied: minSdk overridden to 28 in
android/app/build.gradle.kts with a comment that points back at the
decision register and forbids lowering it without re-opening DEC-004.

DEC-015 applied: shipped English + Simplified Chinese at MVP.
  - pubspec.yaml gains flutter_localizations + intl + generate:true.
  - l10n.yaml emits lib/l10n/generated/AppL10n (no synthetic package
    — that was removed in Flutter 3.41+).
  - lib/l10n/app_en.arb is the source of truth; lib/l10n/app_zh.arb
    mirrors the key set in zh-Hans. ARB metadata explicitly
    reaffirms ADR-008: server-provided content (channel names,
    nicknames, welcome banners) is preserved verbatim and never
    translated.

lib/main.dart and test/widget_test.dart were rewritten from the
template counter into a minimal localized scaffold that proves
both locales render correctly.

Empirical verification:  reports no issues;
 runs the two locale smoke tests and both pass.

crates/chanora_storage/src/lib.rs

@@ -0,0 +1,58 @@
+//! # `chanora_storage`
+//!
+//! Two strictly separated repositories per SAD-067:
+//!
+//! * [`LocalDatabaseRepository`] — non-secret state (bookmarks,
+//!   settings, identity *references*) via SQLite. Crate choice:
+//!   `rusqlite` bundled (DEC-013.1).
+//! * [`SecretStorageRepository`] — secret material (identity private
+//!   keys, server passwords) via platform secure storage. Linux
+//!   policy: Secret Service preferred, kernel keyutils fallback
+//!   (DEC-013.2). Other platforms TBD per SS-TC-001/002/004/005.
+//!
+//! Secret values **never** appear in the local DB (SS-AUD-001/002);
+//! bookmarks store only an `identity_ref` lookup name into the
+//! secure store.
+//!
+//! ## Status
+//!
+//! Scaffold only. `poc/secure-storage-spike` and
+//! `poc/sqlite-storage-spike` will be promoted here.
+
+#![forbid(unsafe_code)]
+#![warn(missing_docs)]
+
+use thiserror::Error;
+
+/// Errors raised by either storage repository.
+#[derive(Debug, Error)]
+pub enum StorageError {
+    /// Looked-up entry does not exist.
+    #[error("not found")]
+    NotFound,
+    /// Schema migration error.
+    #[error("migration: {0}")]
+    Migration(String),
+    /// Underlying SQLite error.
+    #[error("sqlite: {0}")]
+    Sqlite(String),
+    /// Platform secure-storage backend rejected an operation.
+    #[error("secure-store: {0}")]
+    SecureStore(String),
+}
+
+/// Marker trait for the non-secret database side. Concrete impl will
+/// land alongside the `BookmarkRepository` / `SettingsRepository`
+/// traits promoted from the SQLite PoC.
+pub trait LocalDatabaseRepository: Send + Sync {}
+
+/// Marker trait for the platform secure-storage side. Concrete impl
+/// will land alongside the `Secret` newtype + per-platform adapters
+/// promoted from the secure-storage PoC.
+pub trait SecretStorageRepository: Send + Sync {}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_compiles() {}
+}