Meta: Blob Submission Pipeline

## Summary

Tracking issue for the full blob submission (write path) pipeline. This is a prerequisite for all higher-level storage APIs (#43).

Apex is currently read-only. To support S3, Git, and npm storage APIs, we need a complete write path: construct transactions, sign them, manage nonces, broadcast, and confirm.

## Sub-issues (dependency order)

### Core submission client
- [ ] #4 — Evaluate custom tx submission client vs celestia-node client (root decision)

### Signing & identity
- [ ] #9 — Keyring and signing for transaction submission
- [ ] #5 — Multi-account support for transaction submission

### Reliability
- [ ] #8 — Nonce management for transaction submission
- [ ] #18 — Rate limiting and circuit breaker for transaction submission
- [ ] #19 — Transaction confirmation and lifecycle tracking

## Dependency graph

```
#4 (submission client decision)
├── #9 (keyring/signing)
│ └── #5 (multi-account)
├── #8 (nonce management)
│ └── #5 (multi-account)
├── #18 (rate limiting / circuit breaker)
└── #19 (tx confirmation / lifecycle)
```

## Relationship to storage APIs

The storage platform features (#43) all depend on this pipeline:

```
S3 API (#40) ──┐
Git remote (#41) ──┼── Blob Submission Pipeline (this issue)
NPM registry (#42) ──┘ │
┌─────┴─────┐
#4 #8 #9 #5 #18 #19
```

Without a working write path, the storage APIs are read-only caches at best.