Refactor src into focused modules; reduce dead/redundant code

[Copilot]

Splits large flat files into focused modules and removes dead/commented code throughout. Net result: ~87 lines removed, improved encapsulation.

Module reorganisation

• src/nwb.rs → src/nwb/ with data.rs (types) and loader.rs (SQL loading logic)
• src/world.rs → src/world/ with resources.rs (Bevy resources/components) and systems.rs (systems)
• src/lib.rs inline I/O helpers → src/io.rs

Sub-modules are private; only the minimal public API is re-exported:

// nwb/mod.rs
mod data;
mod loader;
pub use data::NWBNetworkData;
pub use loader::preprocess_roadmap;

// world/mod.rs — LoadedMaterials/WorldTracker are internal; removed from pub exports
mod resources;
mod systems;
pub use resources::{WorldConfig, WorldEntity, WorldEntitySelectionType};
pub use systems::convert;

Dead code removed

• FilterResource {} — unused struct
• Commented-out Camera3d spawn block, old config paths, unused plugins in main.rs
• Commented-out process_edges(0, …) call in ui/layers.rs
• Dead feature flags (iter_collect_into, map_try_insert, test) in main.rs binary entry point

Simplifications

spatial.rs — read_f64/read_u32 each repeated 4–8 identical iter.next().ok_or(…)? expressions; replaced with a shared const-generic helper:

fn read_bytes<'a, I, const N: usize>(iter: &mut I) -> rusqlite::types::FromSqlResult<[u8; N]>
where I: Iterator<Item = &'a u8> {
    let mut bytes = [0u8; N];
    for b in &mut bytes {
        *b = *iter.next().ok_or(rusqlite::types::FromSqlError::InvalidType)?;
    }
    Ok(bytes)
}

stopwatch! macro — reduced from 4 arms to 1: the $x:block arm was unreachable (shadowed by $x:expr), and the two print variants were never called.

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