[zhongkechen]

Issue #, if available

Related to aws/aws-durable-execution-docs#295.

Merge order: Merge the shared guide aws/aws-durable-execution-docs#304 first so the new canonical sampling link resolves.

Description of changes

Define the supported sampling modes for durable spans: preserve an explicit upstream Sampled decision, or use a deterministic decision from the canonical trace ID with configuration unchanged across retries and reinvocations. Clarify that non-deterministic local policies are unsupported when no upstream decision exists, and in-memory reuse does not extend that support boundary.

Preserve Python's header → same-trace ambient span → configured sampler precedence. Correct ratio verification and troubleshooting guidance so readers check upstream decisions and whole executions instead of expecting sampled operations to disappear independently.

Design

Documentation only; no design note or simpler code alternative is needed. No runtime source, public API, or test expectations change. The README previously left the support boundary implicit and described ratio verification without its upstream-decision precondition.

Assumptions

Header and ambient precedence, per-invocation resolution, and durable-span decision reuse match durable_sampling.py and the existing sampler tests. The shared guide anchor is introduced by the prerequisite PR above.

Decisions not covered by the issue or the design note

• Update the existing verification and troubleshooting text alongside the sampling section to remove contradictory guidance.
• Rephrase the fallback-anchor example so it does not imply that local decision drift across invocations is supported.
• Run documentation checks without SDK tests because only README prose changed.

Validation

• git diff --check origin/main..HEAD
• Repository commit-message lint
• GitHub Markdown rendering; checked the sampling heading, two-mode list, and local/canonical link structure

Checklist

Runtime branch coverage, regression-first tests, and edited test expectations are not applicable to this README-only change.

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