Refactor: breadcrumb-nonreentrant-upgrade-lifetime-guard

[drmoisan]

• Work Mode: full-feature

Problem / Why

BreadcrumbCoordinatorUpgradeLifetime guards population work by entry-time lease currency, through
TryRunCurrent and RunSynchronous. That verdict is deliberately an entry-time check only: a guarded
action that re-entrantly begins another population, or that calls back into the lifetime, is not
prevented from doing so.

Feature #501 relied on that property and added a test pinning it,
TryRunCurrent_ReentrantInvalidateStillReportsEntryTimeInvocation, so the present behavior is
specified rather than accidental. Research section 6.2 option C proposes the alternative: a guard that
refuses, rather than permits, a nested population under the same lifetime.

This was out of scope for #501. That issue fixed four ordering and lifetime defects (#462, #500, #501,
#502) without changing the guard's re-entrancy contract. Adopting option C changes the contract for
every caller of the lifetime, which carries its own regression surface; folding it in would have
widened a four-defect correctness fix into an API redesign.

Proposed Behavior

Decide whether nested population under a single lifetime is ever legitimate. If it is not, make the
guard non-re-entrant so a nested population is refused, and convert the existing entry-time test into
the negative case.

Acceptance Criteria

• A decision record states whether nested population under one lifetime is legitimate.
• If refused, TryRunCurrent and RunSynchronous reject a nested population deterministically.
• Every TryRunCurrent and RunSynchronous caller is audited for a nested-population path.
• TryRunCurrent_ReentrantInvalidateStillReportsEntryTimeInvocation is updated to match the chosen contract.

Constraints & Risks

• Changes a contract every caller of the lifetime depends on; the blast radius is the whole breadcrumb
  coordinator surface.
• Feature Bug: breadcrumb-hub-postjson-caches-before-broadcast-starves-attachments #501 shipped a test that pins the CURRENT entry-time semantics. That test must be updated
  deliberately, not deleted, or the change will look like a regression.
• Touches files owned by sibling features; coordinate ownership before editing.

Test Conditions

• Unit coverage for a nested population attempt under one lifetime
• Unit coverage for the existing non-nested path, proving no behavior change
• Regression coverage for every audited caller

Source

From: docs/features/potential/2026-08-27-breadcrumb-nonreentrant-upgrade-lifetime-guard.md