refactor: extract diagnostics state

[zatevakhin]

behavior

• extract DiagnosticsState, LogLevel, and AppLogEntry into private src/diagnostics.rs
• replace six flat App fields with one crate-visible diagnostics state while preserving defaults, status strings, log order/deduplication, filtering, cursor behavior, and session-reset behavior
• keep runtime ticks and log elapsed timestamps on the same DiagnosticsState::started_at

compatibility

• retain exactly four logic-free App forwards: push_log, set_status, filtered_logs, and cycle_log_level_filter
• tag all four as phase 11 compatibility forward for later deletion
• add no getter, alias, Deref, re-export, or additional state group

tests

• move and expand focused diagnostics tests for constructor defaults, duplicate suppression and changed entries, status logging, case-insensitive message/target/level filtering, minimum-level filtering, level cycling, and shared elapsed timing
• preserve cross-boundary connection/status, ACP warning, editor, keyboard filter, runtime, and popup rendering coverage
• pass cargo fmt --all -- --check, all-target/all-feature check, Clippy with warnings denied, build, 760 tests, and git diff --check

non-goals

• no connection, session, auth, profile, mesh, navigation, model, command/effect, popup-routing, ACP transport, or UI layout redesign
• no second Phase 4 state group