[ebigunso]

Follow-up to a Copilot finding on the merged #115.

When the currency query failed during stats projection, the structured cause said endpoint hydration had failed, although that read had already succeeded. ADR-I-0029 makes structured causes authoritative, so this was a false typed fact.

The fix is a rename, not a new variant: EndpointHydration becomes GraphRead in both StatsUpdateCause and RetrievalStatsHealthCause, with the message and wire token to match. A consumer acts the same way on any failed graph read that feeds the projection, and the inner graph error already says which read failed.

Evidence: the existing currency-failure regression now checks the returned and retained causes and both wire tokens. Internal Tier D review approved; fmt, check, clippy with warnings denied and the full test suite pass for the worker and independently for the reviewer.

This PR is outside the merges the decider pre-authorized and waits for approval.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟢 Approval recommended

The rename is consistently applied, accurately documented, and covered for both returned and retained serialized causes.

Review effort: Balanced
Findings: None

What changed in this PR

Corrects stats projection failures so structured causes accurately report graph-read failures.

Changes:

• Renames both stats cause variants from EndpointHydration to GraphRead.
• Updates wire tokens, messages, tests, and documentation.
• Verifies currency lookup failures in returned and retained health causes.

File	Description
src/​errors.rs	Renames typed causes and updates conversion logic.
src/​usecases/​stats_projection.rs	Reports all projection graph-read failures consistently.
src/​usecases/​remember.rs	Tests currency failures and serialization tokens.
tests/​write_planning_tests.rs	Updates integration expectations.
README.md	Documents graph-read failure reporting.
docs/​design/​database/​graph_schema_design.md	Records the structured cause contract.

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[ebigunso]

Superseded by #150: this fix was merged into the stack there (with its reconciliation against the scene surfaces the stack added) and reviewed by both tiers. Closing at the decider's direction.