clippy: two pre-existing -D warnings failures (block local validation with strict clippy)

[ajianaz]

What

Two warnings fire under `cargo clippy --all-targets -- -D warnings` on current `develop` (verified identical before any of the open PRs), failing strict clippy locally. CI may pass with a different toolchain, but per AGENT.md this command is the project lint bar.

Findings

1. Duplicated attribute — `src/index/ast.rs:6`: module-level #![cfg(feature = "tree-sitter")] while the declaration in `src/index/mod.rs:7` is already cfg-gated.
2. Collapsible else-if — `src/main.rs` (~line 1161–1186, shifts between branches): an `else { if … }` block can be collapsed to `else if`.

Suggested fix

1. Remove the inner `#![cfg(...)]` attribute from `ast.rs` (or drop the gate on the `mod` declaration — pick one).
2. Collapse the else-if in `main.rs`.

Trivial, no behavior change — good first cleanup PR so toolchains matching CI don't keep reporting drift.

Repro

```bash
cargo clippy --all-targets -- -D warnings
cargo clippy --all-targets --features tree-sitter -- -D warnings
```

[ajianaz]

Verified on develop after #528–#531 landed (4182466):

cargo clippy --all-targets --features tree-sitter -- -D warnings

→ clean on rustc/cargo 1.97.1 — neither finding reproduces on the current toolchain.

The reported code pattern still exists though (src/index/ast.rs carries the inner #![cfg(feature = "tree-sitter")] while the mod ast; declaration in src/index/mod.rs is already cfg-gated), so leaving this open as a minor cleanup: removing the inner attribute would make older clippy versions happy too.