fix(vault): validate on-ledger token balance during init

[iammrjude]

Summary

Closes #153

This PR updates callora-vault so init now validates the vault's on-ledger USDC balance before accepting a positive initial_balance.

What changed

• added an on-ledger token balance check in contracts/vault/src/lib.rs
• init now fails fast with:
  • initial_balance exceeds on-ledger USDC balance
• only positive initial_balance values trigger the token-balance read
• added vault tests that cover:
  • successful init when the vault's token balance covers initial_balance
  • failing init when initial_balance exceeds the actual token balance
• updated existing vault tests that used a positive initial_balance without pre-funding the vault
• synced INVARIANTS.md so the documented init preconditions match the implementation

Why

INVARIANTS.md already states that when a vault is initialized with an internal starting balance, the contract should already hold at least that much USDC on-ledger. This PR makes that invariant explicit in code.

Validation

Ran locally from the workspace root:

• cargo fmt --all
• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo build --target wasm32-unknown-unknown --release -p callora-vault
• stellar contract build --package callora-vault
• cargo tarpaulin --config tarpaulin.toml

Test summary

• revenue pool: 20 tests passed
• settlement: 45 tests passed
• vault: 104 tests passed
• workspace coverage: 99.00% (596/602)

Security notes

• prevents the vault from starting with an internal accounting balance that is not backed by real on-ledger USDC
• preserves fail-fast behavior during initialization
• keeps zero-balance initialization compatible for tests and deployments that intentionally start empty

Reviewer note

This branch is currently stacked on top of the CI fixes from #138 so that the full validation suite stays green before that PR merges. Once #138 is merged, this PR can be retargeted to main and the diff should reduce to the #153 vault/invariant changes.

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[iammrjude]

@greatest0fallt1me

I am done.

All checks have passed. Please review and merge.