docs(lease): define observer reconciliation contract

[smiggleworth]

Summary

• define the shared race-safe Lease inventory observer lifecycle
• standardize periodic reconciliation as clamp(shortest expected lease TTL / 2, 5s, 60s) with +/-20% jitter
• add AC-LEASE-014 with the six common cross-SDK conformance scenarios

Validation

• cargo fmt --all -- --check
• cargo test --locked --workspace (1,607 library tests plus integration and doc tests)
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings -D clippy::pedantic

Supports #219.

[Copilot]

Pull request overview

Defines and cross-references a shared, race-safe Lease inventory observer contract across SDKs, including bootstrap sequencing, periodic reconciliation guidance, and a standardized conformance scenario set to ensure consistent behavior across implementations.

Changes:

• Adds a high-level Lease inventory observer lifecycle spec (subscribe-before-list ordering, invalidation handling, retry/recovery rules).
• Standardizes periodic full-LIST reconciliation guidance as clamp(shortest expected lease TTL / 2, 5 seconds, 60 seconds) with ±20% jitter and a 60s default.
• Introduces AC-LEASE-014 capturing six cross-SDK observer conformance scenarios.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/clients/spec/lease-schedule.md	Adds the detailed observer bootstrap/reconciliation lifecycle and scheduling guidance.
docs/clients/client-requirements.md	Updates normative client requirement REQ-API-004C to include periodic backstop guidance and points to AC-LEASE-014.
docs/clients/acceptance/queue-notice-rpc-lease.md	Adds AC-LEASE-014 enumerating the shared cross-SDK observer conformance scenarios.

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