Rendering makes up 75% of compile time

Background: [Discussion on Discord](https://discord.com/channels/691052431525675048/1489167876085776444/1489179712189235240) with @ChristopherBiscardi and @Elabajaba.

# Bevy version

Tested on main, commit `45e454a83bffd7a496621ba77bc3429fa472733d`.

# System information

- Linux (Pop!_OS), Ryzen 9 9950x3d CPU
- cargo 1.96.0-nightly (888f67534 2026-03-30)

# Reproduction

Run `cargo clean && cargo build --timings --example 3d_scene`, then inspect the generated timings file.

Mine is here for reference: https://gist.github.com/TimJentzsch/3fcc84bc8dfa261f7775ade2b2cb274b

# Results

The flamegraph shows that a large part of the compilation is spent on a waterfall of rendering crates that is largely compiled sequentially.

The critical path here is:

1. `bevy_render`: 25.15s
2. `bevy_core_pipeline`: 17.04s
3. `bevy_pbr`: **43.61s**
4. `bevy_gizmos_render`: 5.4s

These rendering crates sum up to 91.2s, which is 76% of the total compile time.
About half of that is `bevy_pbr`.

# Comparison with Bevy ~0.12

The original discussion was about comparing compile times with an old Bevy version, commit `22e39c4` (v0.12) that @IceSentry [benchmarked back in 2024](https://discord.com/channels/691052431525675048/1245430414533722265/1245430963547148389).
On https://github.com/TimJentzsch/bevy/tree/old-bevy is a small patch of this commit (figured out by @Elabajaba) that still compiles today.

Timings on my system:

Full timings: https://gist.github.com/TimJentzsch/ae10132cc0b6b20f9b74245008183c50

Notably, the compile time is only ~17% of what it is today.
`bevy_pbr` takes over 17 times as long now.

Of course, it's not a fair comparison, Bevy's feature set has grown massively since then, especially with regards to rendering. But still interesting to see the differences.

Reproduce the report with `cargo clean && cargo build --timings --example 3d_scene`, then inspect the generated timings and compare the rendering crates named in the issue: `bevy_render`, `bevy_core_pipeline`, `bevy_pbr`, and `bevy_gizmos_render`. Use the referenced timings and the v0.12 comparison as baselines; done means identifying and addressing the cause of the rendering compilation waterfall or documenting a concrete, measurable improvement.

Written by the indexing model from the issue text.