build: Fix various compiler warnings and uninitialized members

[Joe-Abraham]

This PR fixes various build warnings reported during CI as part of resolving #322.

To make the review process easier, the fixes have been broken down into the following logical commits:

1. Fix unused parameter warnings: Added [[maybe_unused]] to unused parameters across abstract interface implementations and visitors, suppressing compiler warnings.
2. Fix uninitialized member warnings: Added default initializers (e.g., =, = nullptr, or {}) to struct and class members to ensure deterministic initialization.
3. Fix integer sign comparison and loop counter warnings: Transitioned to std::cmp_equal, std::cmp_less, std::cmp_not_equal, etc., from , and ensured loop counters appropriately use size_t where necessary.
4. Fix mixed build warnings: Addressed specific operator precedence warnings (e.g., adding parentheses around || operations in std::isnan checks in literal.cc) and other mixed build issues.
5. Fix redundant std::move warnings: Removed unnecessary std::move calls that prevent copy elision (e.g., on projected->term()) in projection utilities.
6. Fix shadowed function and class/struct friend declaration warnings: Renamed shadowed Equals methods to EqualsSchema, and converted friend class errors to friend struct where applicable.
7. Remove unused functions: Dropped unused local/helper functions from schema_util_test.cc that were triggering unused-function warnings.