chore: make dead code a compile error + drop dead index_types module

[tobyhede]

Summary

Makes dead code a hard compile error workspace-wide, and removes a long-dead module that prompted it.

• [workspace.lints] denying dead_code + unused_imports, with each member crate opting in via [lints] workspace = true. Dead code now fails plain cargo build / cargo test — not only the clippy -D warnings CI job — so an unused fn/field/variant/import can't rot silently.
• Removed tests/sqlx/src/index_types.rs (+ its lib.rs mod and IndexTypes re-export). It was a typo-prevention constants scaffold added with the SQLx test migration (2025-10-27, a252cff) but never adopted — all six constants (HMAC, BLAKE3, ORE64, ORE_CLLW_U64_8, ORE_CLLW_VAR_8, ORE_BLOCK_U64_8_256) had zero references since the commit that created them. The tests use the literal strings ("hm", "ob", …) inline instead.

Why remove it by hand instead of letting the lint catch it?

rustc's dead_code lint exempts unused pub items in a library crate — it assumes a sibling/downstream crate may consume them. eql_tests is a lib whose integration tests (tests/*.rs) are separate crates that link it, so its pub harness items are always "potentially used", and the unused pub module slipped through. Verified: building with -D dead_code did not flag it. The lint guards the private dead-code case going forward; the pub exception is handled by the corollary rule now documented in the workspace Cargo.toml: don't mark a test-support item pub unless an integration test actually consumes it.

Test Plan

• cargo build --workspace clean under the new deny policy (no dead-code/unused violations).
• Every integration test target compiles clean under the policy (cargo test --no-run) — removing index_types broke nothing; no latent dead code to fix.
• eql-scalars / eql-codegen / eql-tests-macros test suites green.
• No code referenced index_types / IndexTypes (0 references, confirmed across full git history).

Notes

• Independent of feat(v3): N-block ORE comparator + ordered numeric & timestamptz #276 (N-block ORE); both branch from eql_v3. Trivial potential overlap in tests/sqlx/Cargo.toml (this adds a [lints] table; feat(v3): N-block ORE comparator + ordered numeric & timestamptz #276 adds the rust_decimal dep) — easily resolved.
• Base is eql_v3, not main.

🤖 Generated with Claude Code

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