feat: add schema versioning and automatic migrate() on upgrade across all contracts (#24)

[Iwayemi-Kehinde]

Summary

Closes #24

Adds an on-chain schema versioning system and automatic migration to all 6 Soroban contracts. When a contract is upgraded via upgrade(), the new WASM's migrate() is self-invoked to keep storage compatible with the new code.

Changes

All 6 contracts

• Added SchemaVersion / SCHEMA_V storage key with CURRENT_SCHEMA_VERSION = 1
• Added get_schema_version() / set_schema_version() storage helpers
• Added MigrationRequired error variant to each error enum
• Added pub fn migrate(env) — reads stored version, applies per-version migration logic, writes current version. Idempotent (safe to call multiple times)
• Modified pub fn upgrade(env, new_wasm_hash) to self-invoke migrate() after updating WASM, using the pattern:

contracts/vendor-registry-contract

• Added upgrade() function (was missing)
• Added set_admin() function (was missing)
• Added SchemaVersion variant to DataKey enum

contracts/vouching-contract

• Added upgrade() function (was missing)
• Added set_admin() function (was missing)
• Added SchemaVersion variant to DataKey enum

contracts/parameters-contract

• Added set_admin() function (was missing)

Conflict Resolution

File	Resolution
parameters-contract/src/errors.rs	Keep upstream's ReentrancyDetected=5, Overflow=6, Underflow=7; add MigrationRequired=8
parameters-contract/src/lib.rs	Keep upstream's reentrancy guards in upgrade(), set_admin(), update_parameters(); add self-invoke migrate() after exit_non_reentrant() in upgrade()
reputation-contract/src/errors.rs	Keep upstream's ReentrancyDetected=7; add MigrationRequired=8
reputation-contract/src/lib.rs	Keep upstream's reentrancy guards in set_admin(), set_updater(), upgrade(); add self-invoke migrate() after exit_non_reentrant() in upgrade(); add migrate() function
vendor-registry-contract/src/errors.rs	Keep upstream's ReentrancyDetected=8, Underflow=9; add MigrationRequired=10
vendor-registry-contract/src/types.rs	Keep upstream's Locked variant; add SchemaVersion variant (order: Admin, Locked, SchemaVersion, Vendor(Address), VendorCount)
vouching-contract/src/errors.rs	Keep upstream's ReentrancyDetected=10, Overflow=11, Underflow=12; add MigrationRequired=13

Testing

• All existing tests pass (no data migration needed for v1 — framework only)
• migrate() is idempotent: calling it twice is a no-op
• Reentrancy guards preserved from fix(safe-math): fix #20 by implementing overflow-safe arithmetic across contracts #29

[EmeditWeb]

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