Retry engine and error classification: instanceof IoError after the flat tier, non-finite delayOverride, backoff NaN #78

Part of the audit remediation umbrella #67. Milestone 4. Severity: MEDIUM. Requirements: RETRY-2, RETRY-11, RETRY-40, §10 item 17.

Current SDK behavior

• The cause-walk misses four I/O leaves. classify.ts:73 narrows with
  current instanceof IoError. After the 2026-09-04 tier flattening, EndOfStreamError,
  SourceContractViolationError, ClosedResourceError and AllocationLimitError
  (io/errors.ts:29,51,67,83) extend DexpaceError directly and are grouped by isIoError.
  Only IoError itself and TransportFailureError match. A truncated response body
  (EndOfStreamError) is not retryable; a no-response transport failure is.
  docs/deviations.md item 17 gives "the cause-walk returns retryable for any IoError" as the
  reason TransportFailureError must extend IoError.
• A non-finite delayOverride is fatal and masks the real failure.
  resolveDelay (retry/engine.ts:127-138) does not validate the override result.
  overshootsBudget(NaN) is false, waitFor's delayMs <= 0 guard (:294) is false,
  Clock.sleep rejects with RangeError, and :416-422 folds it into the terminal failure.
  delayOverride: () => NaN with maxAttempts: 3 gives one send and surfaces
  RangeError: Clock.sleep: durationMs must be a non-negative finite number, got NaN, with the
  real TransportFailureError in .suppressed. A throwing override correctly gives three
  sends (RETRY-40).
• Backoff is NaN when initialDelayMs is 0 and the power overflows.
  backoff.ts:74: 0 * Infinity. retrySettings({initialDelayMs: 0, multiplier: 1e200, maxAttempts: 4})
  gives computeDelay(3) === NaN; so does {initialDelayMs: 0, multiplier: 2, maxAttempts: 2000}
  at attempt 1100. The comment at :49-50 assumes initialDelayMs > 0.

Expected behavior

The retry classifier has one stated rule for I/O failures, and each of the five I/O error
classes has a test that pins whether it is retryable. A delayOverride that returns a
non-finite value is treated like one that throws: the computed backoff is used and the
attempt loop continues. computeDelay returns a finite non-negative number for every
accepted setting and attempt count.

Notes and leads

• instanceof IoError is a decision. (a) Switch to isIoError(current) and decide per leaf;
  AllocationLimitError and ClosedResourceError should probably not retry; an
  EndOfStreamError raised while writing a request body means bytes went out, so the engine's
  replayability gate must hold. (b) Keep instanceof IoError as "only transport-layer failures
  retry" and rewrite item 17 and io/index.ts to say so. Either way, five classify tests.
• resolveDelay: Number.isFinite(override) && override >= 0 ? override : computed.
• computeDelay: short-circuit initialDelayMs === 0 to 0, or Number.isFinite guard
  before Math.min.
• Subtask 5 (typed error surfaced) touches engine.ts too; land that first to avoid a
  conflict in withTrail.
• Patch changeset for @dexpace/core. Update docs/deviations.md item 17.