Feature/ci workflow consolidation and enhancements

[pope-h]

Consolidate CI + enforce lints + test cleanup + view helpers

Closes #67
Closes #68
Closes #69
Closes #70

What changed

CI

• Consolidated overlapping GitHub Actions workflows into a single CI workflow with a matrix:
  • fmt (cargo fmt --all -- --check)
  • clippy (cargo clippy --all-targets --all-features -- -D warnings)
  • tests (workspace tests once; removes duplicate test runs)
  • build-wasm (MVP-compatible build + wasm-opt)
• Added caching for:
  • Cargo registry/git/bin
  • ~/.local/bin (downloaded soroban binary)
  • cargo-installed tools (e.g. wasm-opt)
• Fixed workflow reachability by adding workflow_dispatch + schedule (required by e2e-devnet job conditional)

Tests

• Migrated legacy inline #[cfg(test)] mod tests out of contracts/vault/src/lib.rs into contracts/vault/src/tests/test_legacy_inline.rs
• Ensures cargo test runs through the unified suite in src/tests/ (no split coverage / duplicate suites)

New read-only helpers

• Added:
  • get_user_info(user) -> { principal, shares }
  • preview_deposit_to_shares(amount)
  • preview_shares_to_assets(shares)
• Added tests for:
  • bootstrap behavior
  • yield + rounding consistency against existing conversion helpers

Verification

• cargo test (vault crate) passes
• cargo clippy --all-targets --all-features -- -D warnings passes

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