feat(core): add BackoffManager for transient error backoff (2/3)

[abueide]

Summary

• Adds BackoffManager for global transient error backoff (5xx, 408, 410, 460)
• Adds BackoffConfig, BackoffStateData types to types.ts
• Expands HttpConfig to include backoffConfig
• Adds validateBackoffConfig in config-validation.ts with SDD-specified bounds
• Core test suite (12 tests)

Merge order

Depends on #1153. Rebase onto master after #1153 lands.

1. feat(core): add UploadStateMachine for rate limiting (1/3) #1153 — UploadStateMachine + rate limit types
2. feat(core): add BackoffManager for transient error backoff (2/3) #1152 (this) — BackoffManager + backoff types
3. feat(core): add error classification and default HTTP config (3/3) #1150 — Error classification + defaultHttpConfig

Design decision: Global vs per-batch backoff

The SDD assumes file-based batches with stable identifiers ({writeKey}-{fileIndex}). The RN SDK uses a queue model where chunk() in SegmentDestination.sendEvents() produces new arrays each flush — there is no stable batch identity across flushes.

Solution: BackoffManager tracks global transient error state instead of per-batch metadata. Same exponential backoff formula, but state is global. This is equivalent in practice because during a TAPI outage all batches fail anyway. Within a single flush, all batches are still attempted sequentially per the SDD.

Components

BackoffManager (148 lines)

Global transient error backoff manager.

Methods:

• canRetry() — gates the flush (like UploadStateMachine's canUpload())
• handleTransientError(statusCode) — sets exponential backoff
• reset() — clears on success
• getRetryCount() — returns current count for X-Retry-Count header

States: READY | BACKING_OFF

Backoff formula: min(baseBackoffInterval * 2^retryCount, maxBackoffInterval) + jitter

Config Validation (61 lines added)

Validates and clamps backoff configuration to safe ranges:

• maxBackoffInterval: 0.1s – 86,400s
• baseBackoffInterval: 0.1s – 300s
• maxTotalBackoffDuration: 60s – 604,800s
• jitterPercent: 0 – 100
• maxRetryCount: 1 – 100

Type Definitions (20 lines added to types.ts)

• BackoffConfig — backoff settings from Settings CDN
• BackoffStateData — backoff state persistence shape
• Expanded HttpConfig to include backoffConfig

Test plan

• 12 core tests covering canRetry gate, exponential progression, backoff cap, jitter, max retry/duration limits, disabled config, reset
• devbox run test-unit passes
• No unrelated diffs

[abueide]

Superseded by #1154