fix(campaign): preserve caller cancellation

[drewstone]

Problem

PR #607 forwarded candidate-failure cancellation but replaced a caller-provided cancellation signal.

Change

Compose caller cancellation at the concurrent map boundary. Active candidates receive either cancellation source, and no later candidate starts.

Proof

• focused candidate and concurrency tests: 92/92
• independent regression audit: approved
• full suite: 5,238 passed, 3 skipped
• typecheck, lint, build, packed-package verification: passed
• analyst implementation digest: recomputed and checked

This follow-up must merge before the unreleased #607 behavior is published.

[tangletools]

✅ Auto-approved drewstone PR — 324ba2da

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

This approval is provisional. It rests on the audit running. If the audit cannot run — for example the CLI bridge rejects it — this approval is dismissed rather than left standing, so an unrun check never reads as a passing one.

_{tangletools · auto-approval · reason: drewstone_author · 2026-08-15T04:57:22Z}