test: add the non-blocking 'marathon' E2E tier

Full start-to-finish flows move out of the blocking lanes. E2E_TIERS and
E2ETier gain 'marathon'; describeE2ETier('marathon') is enabled only when
EVALS_TIER=marathon, so the gate/PR and periodic lanes (and the gate census)
never run those cases. The PR profile accepts marathon ids as scheduled
elsewhere and defers them with their own reason, even on full fallback.

test/helpers/e2e-gate.ts

@@ -24,11 +24,12 @@
 
 import { describe } from 'bun:test';
 
-export type E2ETier = 'gate' | 'periodic';
+export type E2ETier = 'gate' | 'periodic' | 'marathon';
 
 /**
  * True when this process should run whole-file-gated paid tests of `tier`:
- * EVALS=1 AND EVALS_TIER exactly equals the tier.
+ * EVALS=1 AND EVALS_TIER exactly equals the tier. 'marathon' cases (full
+ * end-to-end flows) therefore never run in the gate/PR or periodic lanes.
  *
  * Deliberate consequence: EVALS=1 with EVALS_TIER unset is false for BOTH
  * tiers. Tierless runs (`test:evals` / `eval:bg` / `eval:bg:all`) skip every