Key observer dispatches by fan-out lineage

A fan-out instance dispatch or a parallel-branches per-branch
dispatch nested inside an outer fan-out instance was keyed by its
local namespace only, so the same dispatch in different outer
instances shared one key: the second instance's inner nodes
reparented under the first's dispatch and an inner augmentation
reached the wrong outer instance.

Key dispatches in both the OTel and Langfuse observers by their full
enclosing fan-out instance and branch lineage, and resolve a nested
node's parent and find its dispatch node by that lineage too. The
per-branch key reads the branch name from the event so callable
branches still synthesize. Un-defer conformance fixture 039 cases 1
and 2 (nested-lineage augmentation) in both observer harnesses.

Author: chris-colinsky

CHANGELOG.md

@@ -12,6 +12,7 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/). The
 - **Langfuse per-branch dispatch-span observation** (observability §4.3 / §8.4.2, proposals 0042 / 0044). The Langfuse observer now synthesizes a per-branch Span observation under a `parallel_branches` dispatcher node, so each branch's inner observations nest under their own branch span (a three-level dispatcher / per-branch-span / inner-nodes tree) instead of parenting directly under the dispatcher. The per-branch observation carries the OA-emitted `branch_name` alongside the caller baseline metadata and any per-branch augmentation, and the Generation observation now carries `branch_name` too. The OTel observer already produced this shape (proposal 0044 shipped OTel-only in v0.11.0); this brings the Langfuse mapping into line. Callable branches (proposal 0075) are unchanged.
 - **Augmentation no longer lands on a shared-parent fan-out / parallel-branches node** (observability §3.4, proposal 0045). A key set via `set_invocation_metadata` inside a fan-out instance or a parallel-branches branch was incorrectly applied to the shared fan-out / dispatcher NODE span (the fork point) when the augmenting context executed at that node's own namespace, in addition to the per-instance / per-branch dispatch span where it belongs. Both the OTel and Langfuse observers now skip the shared-parent node in that case, matching the behavior already applied to strict-ancestor shared parents. The per-instance / per-branch dispatch spans and the lineage ancestors that carry the augmentation are unaffected.
 - **Langfuse fan-out-instance dispatch nested below the top level** (observability §5.4, proposal 0013). The Langfuse observer's per-instance dispatch synthesis and parent resolution are now prefix-general, so a fan-out node sitting inside a serial subgraph wrapper (rather than at the top namespace level) gets its per-instance dispatch observation synthesized and its inner observations parented under it. This matches the OTel observer, which already resolved across every namespace prefix.
+- **Dispatch spans nested inside an outer fan-out instance no longer collide across outer instances** (observability §5.4 / §3.4, proposals 0013 / 0044 / 0045). A fan-out instance dispatch or a parallel-branches per-branch dispatch sitting inside an outer fan-out instance (a fan-out within a fan-out, or parallel-branches within a fan-out) was keyed by its local namespace only, so the same dispatch in different outer instances shared one key: the second outer instance's inner nodes reparented under the first instance's dispatch, and an inner augmentation reached the wrong outer instance's dispatch. Both the OTel and Langfuse observers now key dispatches by their full enclosing fan-out instance / branch lineage, and resolve a nested node's parent by that lineage too, so each outer instance gets its own correctly-parented dispatch subtree with isolated augmentation. Top-level and serial-nested dispatch behavior is unchanged.
 - **Nested fan-out no longer collapses under concurrency** (engine). A fan-out nested inside an outer fan-out instance shared a single per-fan-out tracking entry across all outer instances, because that entry was keyed by namespace plus node name only. With concurrent outer instances the second instance found the first's entry already marked complete and rolled its result forward, so every outer instance returned the first instance's inner result (silently wrong output) and the inner subgraph ran only once. The tracking key now carries the enclosing fan-out instance lineage, so each outer instance gets its own inner fan-out progress and correct per-instance results. Top-level and subgraph- or branch-nested fan-outs are unaffected (their enclosing lineage is empty). Resume of a fan-out nested inside an outer fan-out instance does not yet round-trip per-outer-instance progress, so it re-runs rather than skipping on resume; tracked as a follow-up.
 
 ## [0.15.0] — 2026-06-22