test: close per-file coverage gaps opened by the message-machine refactor

Downstream packages lost the tests that exercised their agent-loop-facing
edges when the loop was rewritten. Restore 100% per-file coverage with
behavior tests through public seams: llm-retry config validation and
cancellation races, goal replay drift/staleness/teardown edges, plan-mode
disposed-flush, workspace-context empty-change commits, api-proxy
synchronous send failures, acp-snapshot spill-path extraction and refresh
write-back, ACP injection-triggered turns, cli-demo and tui inbox
lifecycle edges, and agent-loop retry/settlement/lifecycle branches.
The only source changes are narrowly-justified v8 ignore annotations on
invariant guards and one redundant-guard removal (workspace-context).

packages/goal/goal/src/fold.ts

@@ -358,6 +358,9 @@ export function applyGoalEvent(state: GoalFoldState, event: SessionEvent): GoalC
     if (source === undefined) return undefined
     // A goal-sourced message without a change must be a positive-round
     // admitted continuation prompt; round zero owes a durable source change.
+    /* v8 ignore next 3 -- decodeGoalEvent returns the change or fails loud for every
+       round-zero goal source, so only positive rounds reach here; the guard keeps
+       replay fail-loud against a decoder change */
     if (source.round === 0) {
       throw new Error(`goal source at session event ${event.seq} lacks goal change data`)
     }

packages/goal/goal/tests/goal.spec.ts

@@ -821,6 +821,12 @@ describe('goal replay validation', () => {
     expect(() => foldGoal(oneChange(change, {
       source: { kind: 'goal', goalId: change.goal.id, revision: 1, round: -1 },
     }))).toThrow('source is invalid')
+    expect(() => foldGoal(oneChange(change, {
+      source: { kind: 'goal', goalId: GoalId('goal-imposter'), revision: 1, round: 0, change },
+    }))).toThrow('mismatched source attribution')
+    expect(() => foldGoal(oneChange(change, {
+      source: { kind: 'goal', goalId: change.goal.id, revision: 2, round: 0, change },
+    }))).toThrow('mismatched source attribution')
     expect(() => foldGoal(oneChange(change, { content: [{ type: 'text', text: 'wrong' }] }))).toThrow('model-visible content')
   })