go/runtime: journal watches used for mapping must use shard context

And not the task term context. The issue is that task term context is
canceled when the term *begins* to change, but we allow the current
transaction to complete, which means writing out documents, which means
we may need to create a new physical partition and are not able to do
that correctly if the journal watch is no longer updating.

Instead, wrap the shard context with a cancel function and cancel
prior watches at the start of the *next* task term.

go/runtime/capture.go

@@ -30,6 +30,7 @@ type Capture struct {
 	restarts     message.Clock         // Increments for each restart.
 	transactions message.Clock         // Increments for each transaction.
 	watches      []*client.WatchedList // Watches of binding journals.
+	watchCancel  context.CancelFunc    // Canceler of watches.
 }
 
 var _ Application = (*Capture)(nil)
@@ -71,12 +72,17 @@ func (c *Capture) RestoreCheckpoint(shard consumer.Shard) (_ pf.Checkpoint, _err
 		return pf.Checkpoint{}, err
 	}
 
-	// Note that prior watches are cancelled with the prior term context.
+	var watchCtx context.Context
 	c.watches = c.watches[:0] // Truncate.
 
+	if c.watchCancel != nil {
+		c.watchCancel() // Cancel watches of previous term.
+	}
+	watchCtx, c.watchCancel = context.WithCancel(shard.Context())
+
 	for _, binding := range c.term.taskSpec.Bindings {
 		c.watches = append(c.watches, client.NewWatchedList(
-			c.term.ctx,
+			watchCtx,
 			shard.JournalClient(),
 			flow.CollectionWatchRequest(&binding.Collection),
 			nil,
@@ -224,7 +230,7 @@ func pollLoop(
 		case op = <-pollCh:
 		}
 
-		// Wait for the prior commit's OpFuture to resolve succesfully.
+		// Wait for the prior commit's OpFuture to resolve successfully.
 		if err := op.Err(); err != nil {
 			return err
 		}

go/runtime/derive.go

@@ -1,6 +1,7 @@
 package runtime
 
 import (
+	"context"
 	"database/sql"
 	"encoding/json"
 	"fmt"
@@ -23,9 +24,10 @@ import (
 // Derive is a top-level Application which implements the derivation workflow.
 type Derive struct {
 	*taskReader[*pf.CollectionSpec]
-	client pd.Connector_DeriveClient
-	sqlite *store_sqlite.Store
-	watch  *client.WatchedList
+	client      pd.Connector_DeriveClient
+	sqlite      *store_sqlite.Store
+	watch       *client.WatchedList
+	watchCancel context.CancelFunc
 }
 
 var _ Application = (*Derive)(nil)
@@ -83,9 +85,14 @@ func (d *Derive) RestoreCheckpoint(shard consumer.Shard) (_ pf.Checkpoint, _err
 		return pf.Checkpoint{}, err
 	}
 
-	// Note the prior watch is cancelled with the prior term context.
+	var watchCtx context.Context
+	if d.watchCancel != nil {
+		d.watchCancel() // Cancel watch of previous term.
+	}
+	watchCtx, d.watchCancel = context.WithCancel(shard.Context())
+
 	d.watch = client.NewWatchedList(
-		d.term.ctx,
+		watchCtx,
 		shard.JournalClient(),
 		flow.CollectionWatchRequest(d.term.taskSpec),
 		nil,