[codex] reduce SonarCloud maintainability findings

[dodok8]

Summary

Reduce the four open SonarCloud maintainability findings tracked in #24 without changing query parsing, query resolution, REPL behavior, or SQLite schema metadata output.

Closes #24

Scope

• Split the parser boolean/arithmetic precedence test assertions into focused AST assertion helpers.
• Split query-resolver expression resolution into narrower compare, null comparison, and membership helpers.
• Split the REPL input loop into line handling, prompt selection, read error handling, and interrupt handling helpers.
• Replace the long SQLiteCatalogFieldRow::new positional constructor with purpose-specific implicit_id, scalar, and link constructors.

Spec and plan alignment

List the spec/ and plan/ files checked for this change. If the change
intentionally diverges from them, explain why and include the required document
update in this pull request.

• spec/query.md: filter expression precedence, null comparison behavior, boolean filters, membership filters, and arithmetic operands are preserved.
• spec/storage-sqlite.md: _engine_catalog_fields metadata shape and stored values are preserved.
• plan/query-expression-model-plan.md: expression model responsibilities remain in parser/resolver layers.
• plan/query-parser-implementation-plan.md: parser precedence test coverage is preserved.
• plan/sqlite-schema-plan-implementation-plan.md: deterministic catalog field row planning and metadata insert contracts are preserved.
• plan/cli-and-tooling-plan.md: REPL behavior remains tooling orchestration and does not add new engine behavior.

Tests

List the automated and manual checks performed.

• cargo test --workspace
• Other: cargo fmt --check
• Other: cargo test -p query-parser parser_preserves_boolean_precedence_with_arithmetic
• Other: cargo test -p query-resolver
• Other: cargo test -p repl
• Other: cargo test -p sqlite-schema-plan

Review notes

Call out API boundaries, temporary rules, migration concerns, or follow-up work
that reviewers should inspect closely.

• This PR is intended as a behavior-preserving refactor for SonarCloud findings only.
• The resolver changes should preserve the existing error behavior for null comparisons and membership item compatibility.
• The SQLite schema plan changes intentionally keep metadata table columns, row values, and deterministic ordering unchanged.
• The REPL changes keep current multiline buffering, exit commands, and two-interrupt exit behavior.

AI assistance

Disclose AI tool usage according to AI_POLICY.md.

• No AI tools were used.
• AI tools were used and are disclosed here and in the relevant commit
  messages with Assisted-by trailers.

Tools and models:

• Codex:gpt-5.5

[sonarqubecloud]

Quality Gate failed

Failed conditions
65.4% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud