chore(dylint): upgrade the Dylint stack to 6.0.1

[zackees]

Context

The Dylint gate on current main still carries the pre-6.0 compatibility stack:

• .github/workflows/dylint.yml installs cargo-dylint and dylint-link 5.0.0.
• The lint crates pin dylint_linting / dylint_testing to trailofbits/dylint revision 4bd91ce7729b74c7ee5664bbb588f7baf30b4a09.
• ci/build_dylint_driver.py clones that revision and builds a custom driver for nightly-2026-03-26.
• The workflow contains an alias-and-retry loop for the old <name>@<toolchain> library naming behavior.

Dylint 6.0.1 is now released and is already in use in sibling repositories. Keeping the old mixed version/git-revision stack preserves expensive bespoke setup and leaves this gate exposed to already-solved rustc-private API drift.

Proposal

Move the entire repository to one documented Dylint 6.0.1 stack:

1. Pin cargo-dylint and dylint-link to 6.0.1.
2. Replace every git-revision dylint_linting / dylint_testing dependency with the 6.0.1 crates.io release.
3. Select one nightly compatible with 6.0.1 and update every Dylint rust-toolchain.toml, workflow reference, and test command together.
4. Remove ci/build_dylint_driver.py, DYLINT_DRIVER_PATH plumbing, and the library alias/retry loop when the 6.0.1-provided driver path makes them unnecessary.
5. Preserve the existing lint implementations, allowlists, stale-allowlist validation, and required status-check behavior.

Acceptance criteria

• Every fbuild Dylint crate uses dylint_linting = "6.0.1" and, where present, dylint_testing = "6.0.1"; revision 4bd91ce... is absent from active configuration.
• CI installs cargo-dylint and dylint-link 6.0.1 explicitly.
• One compatible nightly is pinned consistently across workflow and lint crates.
• The custom cloned-driver build and alias/retry workaround are removed, or a remaining upstream requirement is demonstrated and narrowly documented.
• All Dylint-library formatting/tests pass.
• A single workspace Dylint invocation checks --workspace --all-targets and the required Dylint check is green.
• No lint is disabled and no unrelated allowlist entries are added.

Related issues

• ci(dylint): Dylint job is red on main — alias-loop workaround fails, so NO custom lints are enforced #964 repaired an earlier alias-loop failure.
• fix(ci): restore Dylint after build and package splits #1033 repaired Dylint after the crate splits.