Batch worker fs tree walks into one spawn_blocking per directory

[amankrx]

Description

The worker's input-materialization walks issued a tokio::fs call (each a spawn_blocking on tokio's blocking pool) per file. Materializing an action's input tree touches thousands of files, and at high action concurrency the per-file blocking-pool handoff dominates on many-core hosts while CPU and I/O sit near idle.

Batch both recursive walks to one blocking task per directory:

• hardlink_directory_tree_recursive: read the directory, hardlink files and recreate symlinks inline, create subdirectories, and return them for the async layer to recurse into.
• set_perms_recursive (set_readonly_recursive / set_dir_writable_recursive): convert perms_fn to a synchronous closure and chmod each directory's entries plus the directory itself in a single blocking task, recursing per subdirectory.

Fixes #2609

Type of change

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• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

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Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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Project	Deployment	Actions	Updated (UTC)
nativelink	Ready	Preview, Comment	Jul 23, 2026 12:13pm
nativelink-aidm	Ready	Preview, Comment	Jul 23, 2026 12:13pm

[palfrey]

This is failing due to uses of tokio::task::spawn_blocking and should be using spawn_blocking! instead