[Codeinx]

Overview

No inline string storage keys — all moved to named constants.

What this does

• repayment-waterfall had 4 real inline-literal storage-key call sites (env.storage().instance().has/get/set(&symbol_short!("admin"))) — a typo in any of those silently creates a disconnected new storage slot instead of failing to compile. Added a named ADMIN constant and replaced all 4 call sites. (Test-code uses of symbol_short!("admin") as an identity value passed into initialize(..., admin, ...) are untouched — those aren't storage keys, just test data.)
• pool-manager and invoice-registry already had their keys named, but inside an inline mod storage { ... } block in lib.rs, and invoice-registry additionally had two keys (QUEUED_UPGRADE, UPGRADE_PAUSED) as loose top-level consts outside that module.
• governance already used a type-safe #[contracttype] enum DataKey (arguably the strongest form of this — a typo is a compile error, not a silent new slot) but had it inline in lib.rs rather than its own module.

Consolidated all four into a keys.rs module per crate, as the issue asks, and renamed every reference (storage::X / bare X / DataKey import) accordingly. governance's DataKey also gained Debug, PartialEq derives so its test can assert on it.

Testing

Each new keys.rs has a #[cfg(test)] mod tests asserting every key's underlying Symbol (or, for governance's enum, that its no-argument variants are pairwise distinct) — the acceptance criterion's "tests verify key names."

cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo check --workspace, and cargo test --workspace are all clean.

Closes #6.