[ScriptedAlchemy]

Scope

Refs #4436 — eliminate the split ownership of a stack's current observation and open drift intervals. This is a focused correctness/ownership slice, not full scope-lifetime retirement.

Previously store_observation read the prior snapshot under one mutex, calculated and updated drift under a second mutex, then published the observation after reacquiring the first mutex. Concurrent equivalent provider updates could both calculate drift from the same previous snapshot, emit duplicate transition observations, and race the interval frontier.

This change:

• Replaces the separate observations and drift_intervals maps with a single ScopeObservationState entry holding both under the existing coordinator owner.
• Serializes prior-snapshot comparison, drift update and snapshot publication as one in-memory transition, without holding the lock across network or database I/O.
• Computes new drift intervals before replacing prior intervals, so failed identity derivation leaves the previous intervals unchanged.
• Keeps persisted interval restoration under the same owner.
• Prevents an unavailable policy snapshot prepared before a provider result from overwriting a newly published enabled/degraded provider observation (without changing the behavior of a real provider-unavailable response).

Behavioral regression

concurrent_identical_provider_publications_have_one_drift_transition starts 24 real threads behind a barrier, publishes the same changed stack head, and checks exactly one stack-tip transition and one opening BaseAdvanced interval, plus validation of every emitted drift receipt. This targets the prior read/modify/publish race rather than testing source shape.

Existing typed provider-state, drift close/restore and preflight tests remain.

Verification and limits

• Independently reviewed the exact two-commit, three-file diff against master 91c72aba; no generated contracts or unrelated contributor paths changed.
• At 3f5a11389c, all 89 tests in the macOS Bazel application integration suite passed. The crate formatting target and Clippy for both the production library and test suite passed.
• Disabling and reenabling a provider between observations previously returned its prior enabled snapshot. The regression returned Enabled before the fix and now returns Unavailable until a fresh provider observation arrives. Policy registration, snapshot reuse and final publication now check the same scope state. The previous timestamp still fences late responses.
• Remaining simplify(stack): replace parallel scope registries with one lifetime-owned coordinator state #4436 acceptance: consolidate the policy/circuit registrations and pass a lifetime-owned scope handle, refuse late cleanup from prior registrations, retire state only after the last owner/in-flight settlement, verify durable drift recovery, and measure bounded scope retention. This PR must stay draft until reviewed and validated; it does not close the issue.

No Kev/decision model, release, infrastructure, secret, beta.99 or mixed #4776 changes.

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