Testing: Misfiled unit tests (T-06) #340

Part of the test strategy (captain, 2026-10-05: scenarios that test the changes together; cleaner, faster suites; TESTING.md). Epic #305.

Scope. Move tests/config_tests.rs into src/config.rs tests (dropping cases already there), serialization_tests.rs into src/value, optimizer_tests.rs into src/optimizer, boundary_value_tests.rs into src/value, the parser half of iql_syntax_tests.rs into src/parser/src/statement, the storage half into the storage component binary (T-07).

Acceptance. Five fewer test binaries; no test deleted without a named duplicate in the PR description; cargo test --lib time unchanged within noise.