Improve facet error messages in animint2dir

[ANAMASGARD]

FIXES #168

What's broken

viz <- list(
  plot = ggplot() + 
    facet_wrap(. ~ Species) +  # formula notation
    geom_point(aes(x, y), data = iris)
)
animint2dir(viz) 

• Current error:
  Error: At least one layer must contain all variables used for facetting

Doesn't tell you what's wrong or how to fix it.

• Workaround that works:
  facet_wrap("Species") # string notation works fine

What I am fixing

1. Detect when formula notation is used in facets
2. Give clear error telling user to use string notation instead
3. Fix happens in animint2dir() - animint2-specific code only

Next commit will add the fix to make tests pass.

[ANAMASGARD]

Before :-

Error: At least one layer must contain all variables used for facetting

After :-

Error: Facet variable not found in data: MissingVar
Available columns: Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species
Use string notation like facet_wrap("var") instead of formula notation facet_wrap(. ~ var) ```

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.66%. Comparing base (68b074e) to head (2472155).

Files with missing lines	Patch %	Lines
R/z_animintHelpers.R	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
- Coverage   73.06%   69.66%   -3.40%     
==========================================
  Files         164      163       -1     
  Lines        8758     6033    -2725     
==========================================
- Hits         6399     4203    -2196     
+ Misses       2359     1830     -529     

Flag	Coverage Δ
javascript	?
r	69.66% <96.55%> (+0.12%)	⬆️

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[tdhock]

No obvious timing issues in HEAD=fix-168-facet-error-message

Generated via commit 2472155

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 53 seconds
Installing different package versions	58 seconds
Running and plotting the test cases	3 minutes and 30 seconds

[ANAMASGARD]

Sir @tdhock The 2 failing tests in JS_coverage are in test-compiler-ghpages.R (GitHub Pages tests), not related to this PR. This is the same race condition issue where parallel CI jobs conflict over the shared test repository.

All facet-related tests pass:

• R_coverage: 2096+ tests passed
• CRAN: passed
• My 3 new facet error message tests pass

The fix is working correctly. Please review and give your feedback Sir .