Closes #862

Summary

Adds senior/junior invoice tranching with waterfall repayment and first-loss
allocation, spanning a new Soroban contract, SDK bindings, indexer aggregation,
and investor/admin frontend surfaces. Junior capital absorbs losses first;
senior capital receives priority repayment up to a time-proportional yield cap.

What's included

Contract (contracts/tranche/)

• calculate_waterfall_split and calculate_loss_allocation as pure functions
• Deposit/withdraw with senior advance-rate enforcement, invoice funding,
  waterfall repayment distribution, and loss allocation
• Reentrancy guard on state-mutating entrypoints
• Config admin (set_tranche_config, open_tranche_for_token) and read views
  (get_pool, get_position, get_effective_apy, simulate_waterfall)

SDK (packages/sdk, sdk/)

• client.tranche namespace on AsteraClient: deposit, withdraw,
  getPool, getPosition, getTotals, getEffectiveApy, getInvoiceExposure,
  simulateWaterfall, setConfig
• TrancheClient with real Soroban invocations (replaces the prior stubs)
• trancheContractId added to AsteraConfig

Indexer (indexer/)

• Routes tranche deposit/withdraw/fund/repay/default/config events
• Derived tranche_apy table: realized return per tranche per token, recomputed
  from fund/repay/default events after each ingest batch
• GET /tranches/apy and GET /tranches/:token/apy endpoints

Frontend (frontend/)

• app/invest/tranches: side-by-side senior/junior cards (target vs.
  trailing-realized APY from the indexer), risk explainer, live deposit flow
• app/portfolio: per-investor tranche-position section, hidden when the
  investor has no tranche exposure
• app/admin/tranches: on-chain config editor
• app/admin/waterfall-simulation: hypothetical-default simulation tool

Testing

• 33 contract tests passing (unit, property-based, dust/rounding boundary,
  end-to-end scenarios)
• Property tests cover: split sum invariant, senior-cap ceiling, monotonicity,
  loss sum invariant, junior-first ordering
• E2E scenarios: junior absorbs a full default with senior kept whole; senior
  takes loss once junior is exhausted
• Legacy (non-tranched) pool tests unchanged and still passing

Acceptance criteria

• calculate_waterfall_split / calculate_loss_allocation pure + unit +
  property tested (zero junior balance, exact-threshold, dust/rounding)
• E2E: junior absorbs full default, senior whole; senior takes loss after
  junior exhausted
• Legacy pool deposit/withdraw/repay unchanged; existing tests pass
• Tranche page shows trailing-realized junior APY sourced from indexer data

Notes

Merged latest main (picks up the #864 access-control work); resolved
astera-client.ts and types.ts to keep both the tranche and
accessControl namespaces.