fix: use _fvar._ instead of _ for anonymous fvars

[kim-em]

This PR fixes pp.fvars.anonymous to display loose free variables as _fvar._ instead of _ when the option is set to false. This was the intended behavior in #12688 but the fix was committed locally and not pushed before that PR was merged.

🤖 Prepared with Claude Code

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase feea8a7611f4ec0ad13f72921f87764e4a3937bc --onto 87ec768a509c45987a7d73258da0cc1ef047925c. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-01 10:13:20)

[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 feea8a7611f4ec0ad13f72921f87764e4a3937bc --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-03-01 10:13:22)

[github-actions]

The backport to releases/v4.29.0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-releases/v4.29.0 releases/v4.29.0
# Navigate to the new working tree
cd .worktrees/backport-releases/v4.29.0
# Create a new branch
git switch --create backport-12745-to-releases/v4.29.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ec565f3bf7a3985b6b8592f5cb5fa063b86a0ecf
# Push it to GitHub
git push --set-upstream origin backport-12745-to-releases/v4.29.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-releases/v4.29.0

Then, create a pull request where the base branch is releases/v4.29.0 and the compare/head branch is backport-12745-to-releases/v4.29.0.