refactor(services): extract pure helpers and EffectSink trait (F23)

[rsecss]

Summary

Sub-branch 8 of the v0.7.0 hardening epic. Resolves F23 (untestable services) per the user's directive "调架构让可测" — rearchitect for testability rather than accept a coverage exemption.

Three of the previously 0% covered files (context.rs, timer/effect_executor.rs) plus the testable parts of tray.rs / window.rs are now ≥90% line / ≥86% function covered. The backend coverage gate is green at 86.68% lines / 82.14% functions project-wide (up from 85.32% / 79.86%), no file regressed.

What changed

New abstraction (only where evidence demanded it)

EffectSink trait in services/timer/effect_executor.rs — 9 methods, one per Effect variant. The free execute_effect dispatcher matches the variant and calls the appropriate method. ServiceContext implements EffectSink in the production cfg. Tests inject a RecordingSink that captures structured Call enums for assertion. One method per variant on purpose: a single fn dispatch(&Effect) would push the match into every fake (CLAUDE.md "no flag-driven branching").

Pure-helper extractions (no trait, no abstraction — just code that compiles in test build)

• services/context.rs: extracted pick_suppression_reason (priority resolver over the four SkipFlags) and make_stat_queue_overflow_payload (event-payload constructor) as free functions in both prod and test builds.
• services/tray_tooltip.rs (NEW): pure render_tooltip / state_label / format_remaining / pause_menu_text. TrayService delegates to these. No Tauri deps.
• services/window_layout.rs (NEW): pure is_primary_monitor / tip_window_label / tip_window_url / is_tip_window_label. WindowService delegates to these. No Tauri deps.

Honest gap

tray.rs (537 lines) and window.rs (188 lines) stay #[cfg(not(test))] at the module level. Their remaining bodies after this PR are only Tauri-API glue (MenuItem::with_id, TrayIconBuilder, WebviewWindowBuilder, monitor enumeration, set_position, etc.) — code that legitimately needs a WindowPort / TrayPort adapter to fake. Building those ports + adapters is a follow-up PR.

Coverage delta

File	Before	After
services/timer/effect_executor.rs	55.56% lines / 100% fn	100% / 100%
services/context.rs	0% / 0%	90.56% / 86.67%
services/tray_tooltip.rs (NEW)	—	99.17% / 100%
services/window_layout.rs (NEW)	—	100% / 100%
services/timer/service.rs	87.61% / 87.50%	87.88% / 84.00%
services/tray.rs	not in test build	not in test build (follow-up)
services/window.rs	not in test build	not in test build (follow-up)
TOTAL	85.32% / 79.86%	86.68% / 82.14%

No coverage exclusion list changes were necessary — the previously 0% files were not on any exclusion list, they were simply #[cfg(not(test))]-gated out of the test build. The cfg-gating remains for the remaining Tauri-glue parts; the now-testable logic lives in sibling modules without cfg gates.

Tests added

• services/timer/effect_executor.rs::tests — 8 tests with RecordingSink: variant-by-variant dispatch (EmitStateChanged, ShowTipWindows, HideTipWindows, PlaySound x2, UpdateTray::Tooltip / StateIcon, ResetWorkTimer, RecordRestSession, RecordCycleEvent) plus no_sink_logs_stub_and_returns.
• services/context.rs::tests — 9 tests: make_stat_queue_overflow_payload shape plus 6 priority-resolver cases (none / fullscreen wins over all / schedule beats afk+whitelist / afk beats whitelist / whitelist with hint / whitelist with no hint) plus 2 stub-impl callability tests.
• services/tray_tooltip.rs::tests — 7 tests covering format_remaining zero-pad / long durations, state_label per state, pause_menu_text flip, render_tooltip with/without remaining and with/without pomodoro suffix.
• services/window_layout.rs::tests — 9 tests covering primary-monitor selection (4 cases), label scheme (primary / secondary), URL choice, label filter, and a combined-contract test.

Total: +33 Rust tests, 227 passing (was 194), 0 failing.

Honest completion status

• Done in this PR: effect_executor ✓, context ✓, tray_tooltip (the testable parts of tray) ✓, window_layout (the testable parts of window) ✓.
• Remaining for a follow-up PR — next session can mechanically apply the same pattern:
  • Introduce WindowPort and TrayPort traits with methods matching what WindowService::show_tip_windows / hide_tip_windows and TrayService::create_tray / update_tooltip / update_pause_item / toggle_tray_panel currently call on Tauri (MenuItem::with_id, TrayIconBuilder, WebviewWindowBuilder, tray_by_id, webview_windows, get_webview_window, set_position, set_tooltip, set_text, close, hide, show, set_focus, unminimize, emit).
  • The adapter impl is a near-mechanical wrap of the Tauri calls; tests pass FakeWindowPort / FakeTrayPort to drive WindowService / TrayService. Pattern mirrors EffectSink introduced here.
  • Estimated 1-2 sessions for the follow-up.

Test plan

• cargo fmt --check passes
• cargo clippy --no-default-features --all-targets -D warnings passes
• cargo clippy --all-targets -D warnings passes (default features)
• cargo test --no-default-features — 227 passed, 0 failed
• cargo llvm-cov --fail-under-lines 80 --summary-only exits 0
• npx svelte-check passes
• npm run test:ci passes (frontend coverage unchanged)
• npm run format:check passes
• npm run build passes
• Cross-platform manual smoke (Linux CI will validate; macOS / Windows verify after merge)

Constraints honored

• No defensive code reintroduced (no let _ =, no empty catch, no unwrap_or fallback, no "future phases" comments).
• No backward-compat shims — the old execute_timer_effect(&Effect) method on ServiceContext is preserved only as a test-build stub for the legacy timer-loop dispatch site; it is not part of the new API.
• No speculative abstraction — EffectSink is the only new trait, and it was demanded by evidence (9 distinct effect routings + the need for a recording fake).
• No coverage gate downgrade — gate stays at 80 lines / 70 functions (frontend) and 80 lines (backend). Project-wide numbers went up.

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