fix: Add storage-tier documentation for storage [b#015]

[Jenks00]

Description

This PR addresses issue #1105 for the GrantFox FWC26 campaign. It introduces the contracts/storage package along with comprehensive documentation and focused tests detailing the storage tier architecture (Instance, Persistent, Temporary) used across the project.

Changes Included

• Storage Contract: Created the callora-storage workspace package demonstrating all three Soroban storage tiers.
• Documentation: Added contracts/storage/docs/storage.md detailing the rationale for mapping keys to specific tiers to optimize ledger footprint and avoid silent state archiving.
• NatSpec Rustdocs: Added clear /// rustdoc comments on every StorageKey variant in lib.rs defining the tier and rationale.
• Security & Validation:
  • require_auth() enforced on all state-modifying entrypoints.
  • Overflow-safe math (checked_add) used throughout — no unwrap() on fallible paths.
• Testing: Focused test suite (src/test.rs) covering authorization rejection, state mutation across all three tiers, and overflow safety.

Storage Tier Mapping

Key	Tier	Rationale
Admin	Instance	Small shared config — bumped automatically with every contract interaction
UserBalance(Address)	Persistent	Per-user financial state that must survive across ledger epochs with explicit TTL tracking
RequestMarker(u64)	Temporary	Short-lived deduplication marker; expires after the idempotency window with no ledger bloat

Verification

• cargo test -p callora-storage passes with no failures.
• No regressions introduced; linting constraints verified.

Closes #1105

[greatest0fallt1me]

Closing this as invalid — it doesn't link to a DripsWave-assigned issue or the PR author isn't the current assignee.

To contribute: apply on DripsWave, wait for assignment, then open a PR with Closes #<issue-number> in the body. Thanks!