docs: explain why balances are interpolated

[Yungmon22]

Overview

This PR documents why account balances are interpolated locally between fetches instead of being polled from the API every second, and records the trade-off between accuracy and load that motivates the approach. It also notes how often authoritative data is fetched and clarifies that the ledger clock is the source of truth.

Related Issue

Changes

📝 Balance Interpolation Documentation

• [MODIFY] docs/README.md

  • Adds a section explaining that balances advance locally between fetches rather than being polled every second, and why this deliberate trade-off between accuracy and load was chosen.
  • States how often authoritative balance data is fetched.
  • Notes that the ledger clock is the source of truth for balance progression.

• [MODIFY] hooks/use-accrual.ts

  • Adds inline comments documenting the interpolation behavior and its rationale, so the reasoning is recorded alongside the implementation.

• [MODIFY] lib/api.ts

  • Adds inline comments clarifying the fetch cadence for authoritative data and the ledger clock as the source of truth.

Verification Results

npm run lint
✅ passed

npm run typecheck
✅ passed

npm test
✅ passed

npm run build
✅ passed

Acceptance Criteria	Status
The documentation explains why balances are interpolated locally.	✅ Documented in docs/README.md and inline comments
It states how often authoritative data is fetched.	✅ Fetch cadence documented
It notes the ledger clock is the source of truth.	✅ Noted in docs and code comments

Closes #334

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[Glittersup]

Please resolve conflicts @Yungmon22