feat: allow System.FilePath.walkDir to not follow symlinks

[bibajz]

This PR introduces a new argument to System.FilePath.walkDir - followLinks - which allows the callers to not follow symbolic links and is set to false as default.

---

Closes #10972

---

Rationale:

System.FilePath.metadata action follows symlinks, therefore the branch | ok { type := .symlink, .. } was in fact unreachable. We can leverage the symlinkMetadata action introduced in add3e1a which uses the lstat(2) system call and does not follow symlinks.

This gives us the option to not follow symlinks and expose ourselves to an infinite recursion when traversing a directory structure containing a cycle - a situation that is currently unavoidable.

---

Rationale for setting the followLinks := false default:

I realize that setting the default as false is a breaking change, and I don't know the internal usage of walkDir well enough to see if it breaks anything - from grepping and glancing at the code, not following symlinks seems OK.

I consulted implementations of walkDir in other languages and

• Python stdlib os.walk - https://docs.python.org/3/library/os.html#os.walk does not follow links by default
• Rust crate walkdir - https://docs.rs/walkdir/latest/walkdir/struct.WalkDir.html#method.follow_links does not follow links by default
• Go stdlib filepath/WalkDir - https://pkg.go.dev/path/filepath#WalkDir does not follow links at all.

[bibajz]

Introduced in add3e1a - as per the docstring of IO.FS.FileType :

  /--
  Symbolic links that are pointers to other named files. Note that `System.FilePath.metadata` never
  indicates this type as it follows symlinks; use `System.FilePath.symlinkMetadata` instead.
  -/

it means the symlink branch could never be hit!

[bibajz]

I do not know if it's a real issue, but these lines suffer from TOC/TOU, the first one is:

1. realPath after symlinkMetada may fail if the symlink vanished
2. isDir after realPath may fail if the resolved file vanished

to handle them both, it means we need to cast it to toBaseIO and catch the exceptions individually.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 77ddfd49e676cf723912e70f1e97aa860aa095ab --onto 97d63db52cd2fc3353f8a75f48209de0011f1bd3. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-26 18:10:07)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 77ddfd49e676cf723912e70f1e97aa860aa095ab --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-26 18:10:08)