docs: record testing tiers (ADR 0009), link from 0008 (task 0003)

Add ADR 0009 defining the unit / integration / e2e tiers by the seam
each exercises, with the value-returning vs process-side-effecting line
as the boundary between in-process and e2e-only. Add one line to ADR
0008 noting the Result seam is what makes error propagation assertable
in-process, cross-linking 0009.

.claude/adr/0008-errors-as-values-at-cli-boundary.md

@@ -21,6 +21,7 @@ It keeps the bin layer consistent with core's total-function stance, so the whol
 The possibility of failure becomes visible in a function's signature instead of hidden behind a `throw`.
 The success value cannot be read without first handling the error case, which removes a class of mistakes at compile time.
 A tagged-union error type gives exhaustive handling: a new failure mode is a new tag that every match must account for.
+Because failure is a returned value rather than a side effect, error propagation can be asserted in-process by the integration tier, without spawning the binary (see ADR 0009).
 
 ## Consequences