feat(cron): add Shutdown for graceful drain and harden scheduler lifecycle

CHANGELOG.md

@@ -24,6 +24,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `EventHooks` struct with `OnJobStart` and `OnJobComplete` callbacks, set via
   `WithEventHooks` option.
 - `ErrorFunc` type and `WithOnError` option for error-only callbacks.
+- `Shutdown(ctx context.Context) error` for graceful scheduler draining without
+  cancelling contexts already handed to running jobs.
 - `example_test.go` with runnable examples for pkg.go.dev.
 - `context.Context` threading throughout the public API:
   - `Job.Run(ctx context.Context) error` — jobs receive a cancellable context
@@ -33,6 +35,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     `ErrAlreadyRunning` if a scheduler is already active.
   - `Stop(ctx context.Context) error` — cancels the scheduler, waits for
     in-flight jobs bounded by `ctx`.
+  - `Shutdown(ctx context.Context) error` — stops future scheduling and waits
+    for in-flight jobs without cancelling their contexts.
 - `Clock` interface (`Clock`, `Timer`) and `WithClock` option for deterministic
   testing without `time.Sleep`.
 - `ErrAlreadyRunning` sentinel error returned by `Run` when called twice.
@@ -50,6 +54,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `Entry.WrappedJob` is now unexported (`wrappedJob`).
 - `FuncJob` signature: `func(context.Context) error` (was `func()`).
 - Minimum Go version: **1.26**.
+- `@every` rejects non-positive durations while still rounding accepted
+  sub-second intervals to one second.
+- Malformed `TZ=` / `CRON_TZ=` prefixes now return parse errors instead of
+  panicking.
+- Passing nil to `WithLocation`, `WithParser`, `WithLogger`, or `WithClock`
+  preserves the package defaults.
 
 ### Removed
 

MIGRATION.md

@@ -75,6 +75,15 @@ Returns `cron.ErrAlreadyRunning` if a scheduler is already active.
 `Stop` cancels the scheduler and waits for in-flight jobs, bounded by `ctx`. It
 returns `ctx.Err()` if the deadline elapses before all jobs finish.
 
+### Shutdown (graceful drain)
+
+If you want to stop scheduling new work without cancelling contexts already
+handed to running jobs, use:
+
+```go
+err := c.Shutdown(ctx)
+```
+
 ## 4. Logger — `log/slog` replaces custom interface
 
 The custom `Logger` interface, `PrintfLogger`, and `VerbosePrintfLogger` are
@@ -131,6 +140,13 @@ if errors.Is(err, cron.ErrPanic) {
 }
 ```
 
+`Recover` is not enabled by default. To preserve v3-style panic recovery,
+install it explicitly:
+
+```go
+cron.New(cron.WithChain(cron.Recover(logger)))
+```
+
 ## 9. Removed symbols
 
 | Removed                  | Replacement                |
@@ -149,7 +165,8 @@ if errors.Is(err, cron.ErrPanic) {
 
 1. Update import path to `github.com/hyp3rd/cron/v4`.
 2. Add `context.Context` parameter and `error` return to all `Job` implementations and `FuncJob` / `AddFunc` closures.
-3. Pass a `context.Context` to `Start`, `Run`, and `Stop`.
+3. Pass a `context.Context` to `Start`, `Run`, and `Stop`, and use
+   `Shutdown(ctx)` when you need graceful draining instead of cancellation.
 4. Replace `Logger`/`PrintfLogger`/`VerbosePrintfLogger` with `*slog.Logger`.
 5. Rename `NewParser` calls to `NewSpecParser`.
 6. Replace `Entry.WrappedJob` usage with `Entry.Job`.

README.md

@@ -43,25 +43,28 @@ func main() {
     ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
     defer stop()
 
-    c.Start(ctx)
+    c.Start(context.Background())
 
     <-ctx.Done()
-    c.Stop(context.Background())
+    c.Shutdown(context.Background())
 }
 ```
 
 ## Features
 
 - **Standard 5-field cron expressions** (minute, hour, dom, month, dow) plus
   optional seconds via `WithSeconds()`.
-- **`context.Context` throughout** — `Start`, `Run`, `Stop`, and every `Job`
-  receive a context for cancellation and deadlines.
+- **Context-aware lifecycle** — `Start`, `Run`, `Stop`, `Shutdown`, and every
+  `Job` participate in cancellation and deadlines.
 - **`log/slog` logging** — structured, leveled logging out of the box. Default
   level is `slog.LevelWarn` to keep the scheduler quiet.
 - **`Clock` interface** — inject a fake clock via `WithClock` for deterministic,
   zero-`time.Sleep` tests.
 - **Job wrappers** — `Recover`, `SkipIfStillRunning`, `DelayIfStillRunning`,
   `Timeout`, `MaxConcurrent`, `RetryOnError`, and custom `JobWrapper` chains.
+- **Defensive configuration** — malformed `TZ=` / `CRON_TZ=` prefixes and
+  invalid `@every` intervals return parse errors; nil `With*` options keep
+  defaults.
 - **Named entries** — `AddNamedFunc` / `AddNamedJob` attach human-readable
   labels for logging and observability.
 - **Event hooks** — `WithEventHooks` for `OnJobStart` / `OnJobComplete`
@@ -98,6 +101,10 @@ func main() {
 @every 1h30m
 ```
 
+`@every` accepts positive `time.ParseDuration` values. Durations smaller than a
+second still round up to 1 second; `@every 0s` and negative durations are
+rejected as configuration errors.
+
 ### Time zones
 
 ```go
@@ -106,8 +113,26 @@ cron.New(cron.WithLocation(time.UTC))
 c.AddFunc("CRON_TZ=Asia/Tokyo 0 6 * * ?", myJob)
 ```
 
+Malformed timezone prefixes such as `CRON_TZ=` or `CRON_TZ=UTC` without a
+schedule body return parse errors instead of panicking.
+
+## Lifecycle
+
+Use `Start(ctx)` for a background scheduler and `Run(ctx)` for a blocking one.
+The context passed to `Start` or `Run` is also the parent context for every job.
+
+- `Stop(ctx)` cancels the scheduler and cancels contexts already handed to
+  running jobs before waiting for them to return.
+- `Shutdown(ctx)` stops future scheduling and waits for running jobs to finish
+  without cancelling their contexts.
+- If the `Start` / `Run` context is cancelled directly, both the scheduler and
+  job contexts are cancelled.
+
 ## Job wrappers / Chain
 
+Panic recovery is available but not enabled by default. Install `Recover`
+explicitly if you want panics turned into logged `ErrPanic` errors:
+
 ```go
 c := cron.New(cron.WithChain(
     cron.Recover(logger),
@@ -186,6 +211,11 @@ c := cron.New(cron.WithClock(fakeClock))
 
 See `clock.go` for the interface definition.
 
+## Option defaults
+
+Passing `nil` to `WithLocation`, `WithParser`, `WithLogger`, or `WithClock`
+keeps the package default instead of leaving the scheduler in an invalid state.
+
 ## Migration from robfig/cron/v3
 
 See [MIGRATION.md](MIGRATION.md) for a step-by-step upgrade guide.