feat: support for rustdoc mergeable cross-crate info

[weihanglo]

What does this PR try to resolve?

This is an unstable feature that we designed to fix several performance problems with the old system:

1. You couldn't easily build crate docs in hermetic environments. This doesn't matter for Cargo, but it was one of the original reasons to implement the feature.
2. We have to build all the doc resources in their final form at every step, instead of delaying slow parts (mostly the search index) until the end and only doing them once.
3. It requires rustdoc to take a lock at the end. This reduces available concurrency for generating docs.

Part of

• https://github.com/rust-lang/rust/issues/130676
• https://github.com/rust-lang/cargo/issues/16306

How to test and review this PR?

Design decisions and questions:

• A new intermediate build directory target/debug/docdeps/ shared between all rustdoc invocations: https://github.com/rust-lang/cargo/pull/16309#discussion_r2572066545
• Doesn't take cargo clean into account yet: https://github.com/rust-lang/cargo/pull/16309#discussion_r2570354695
• A new FileFlavor::DocParts is added: https://github.com/rust-lang/cargo/pull/16309#discussion_r2570353860
• One doc-merge per target platform (and run in serial) https://github.com/rust-lang/cargo/pull/16309#discussion_r2570576487
• A new status "Merging" is added. Example output:

     Documenting ...
     Documenting cargo v0.94.0 (/Users/frodo/cargo)
     Finished `dev` profile [unoptimized + debuginfo] target(s) in 29.68s
      Merging 300 docs for host
     Finished documentation merge in 2.74s
    Generated /Users/frodo/cargo/target/doc/cargo/index.html
  

• Who should garbage collect unneeded crate docs under mergeable CCI mode, rustdoc or cargo? (or don't, we shouldn't do any garbage collection?) https://github.com/rust-lang/cargo/pull/16309#discussion_r2570485940

Simple benchmark

• Tested against this PR on top of rust-lang/cargo@15fde07882a7480c3cdeb977c13099f5cd6ee324
• rustdoc 1.93.0-nightly (1be6b13be 2025-11-26)
• macOS Sequoia 15.7.1
• Apple M1 Pro 2021
• 32GB RAM; 10 cores
• 300 crate doc merge
• Test steps:

  argo check
  argo doc
  argo clean --doc && rm -rf target/debug/doc-parts/
  argo doc -Zrustdoc-mergeable-info
  argo clean --doc && rm -rf target/debug/doc-parts/
  

With -Zrustdoc-mergeable-info:

• <40s: ~30s documenting time + 3-5s merge time
• 76262 files, 970.5MiB total

Without -Zrustdoc-mergeable-info:

• ~500s: 8m10s - 8m30s
• 75242 files, 932.8MiB total