[jdjioe5-cpu]

Closes #7

Summary of Changes

1. Environment Configuration (\packages/validation):

   • Added
     onNegativeIntegerSchema\ to validate non-negative safe integers.
   • Added optional \INDEXER_START_LEDGER\ parsing to \parseIndexerEnv\ falling back to
     ull\ and reporting invalid values consistently with existing settings.
   • Added unit tests in \packages/validation/test/env.test.ts\ verifying valid integers, trims, empty strings, and reporting of invalid formats.

2. Indexer Configuration & Precedence (\services/indexer):

   • Updated \IndexerConfig\ in \services/indexer/src/config.ts\ to carry \startLedger: number | null.
   • Introduced pure function
     esolveStartCursor(checkpoint, configuredStartLedger, tipLedger)\ in \services/indexer/src/watcher.ts.
   • Enforced precedence:
     1. Stored checkpoint always wins if present (cold-start config never rewinds or displaces an active cursor).
     2. Configured \INDEXER_START_LEDGER\ is used on a cold start when no checkpoint is stored.
     3. Unconfigured cold starts continue to fall back to the ledger tip via \server.getLatestLedger().
   • Logged exact start source (\rom checkpoint cursor, \rom configured ledger, or \rom ledger tip).
   • Skipped unnecessary \getLatestLedger()\ RPC round-trip when checkpoint or configured start ledger is present.

3. Documentation:

   • Updated .env.example\ with documented \INDEXER_START_LEDGER=\ setting.

4. Automated Testing & Verification:

   • Added tests in \services/indexer/test/watcher.test.ts\ verifying precedence rules, cold-start configuration, checkpoint retention, and logging.
   • All workspace tests (\pnpm test), \pnpm run lint, and \pnpm run typecheck\ passed cleanly.