[foodprocessor]

What type of Pull Request is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Describe your changes in brief

file_cache measured its cache directory with common.GetUsage (du on Linux, a directory walk on Windows) on every StatFs, every eviction-policy pass and, when hard-limit is set, every write that grows a file. On large caches that is a full directory scan in the write path.

This PR adds common/cache_size.Tracker, which keeps a running byte count that file_cache updates as files are downloaded, written, truncated, renamed and deleted:

• The tracker corrects drift by measuring the cache at most every five minutes. Drift can come from, for example, concurrent writes that extend the same file.
• exceedsHardLimit now takes the growth in bytes. Callers compute it from the cached file's current size, and appends use the file size as the write offset.
• The eviction policy reads the tracker instead of measuring the cache, so the duPresent fallback is gone.
• On Windows, GetUsage now returns an error when the directory walk fails instead of silently reporting a partial size.

Behavior to watch: between corrections, the tracked size can differ from the real disk usage. Changes made to the cache directory outside cloudfuse aren't seen until the next correction.

This doesn't depend on #1049 or #1123. #1049 adds the same common/cache_size package (tiered_storage uses it) and the same util_windows.go change, so merging this first lets #1049 merge main without conflicts.

Checklist

• Tested locally
• Added new dependencies
• Updated documentation
• Added tests

Validation (Linux; Windows checked by cross-compiling):

• go build -tags fuse3 ./... and GOOS=windows go build ./...: passed
• go vet -tags fuse3,unittest ./common/cache_size/ ./component/file_cache/ and the GOOS=windows equivalent: passed
• go test -count=1 ./common/cache_size/ ./component/file_cache/ --tags=unittest,fuse3: passed
• golangci-lint run --build-tags fuse3 --new-from-rev=origin/main ./...: 0 issues (Linux and GOOS=windows)

New tests cover the tracker (clamping at zero, summing several paths, a missing path, the reconcile interval), cache-policy deletes, file_cache usage accounting across create, write, truncate, rename, delete and download, and the hard limit across several files. Appends have no dedicated test.

Related Issues

• Related Issue Feature/tiered storage #1049
• Closes #

[jfantinhardesty]

LGTM!