fix: multi-line facet panel strip labels for facet_grid and facet_wrap (#262)

[ANAMASGARD]

Fixes #262 — animint2 currently joins multi-variable facet strip labels onto one line (cyl: 4; am: 0). This PR makes them render on separate lines like ggplot2 does.

Root cause

Facet type	Bug location	Current join
facet_grid	R/z_facets.R build_strip()	"; "
facet_wrap	R/z_facets.R getStrips.wrap()	", "

draw_strip() in animint.js renders one flat <text> node — no <tspan> support today.

Two-commit structure

Commit 1 (this commit) - failing test only

• Adds test-renderer1-facet-multiline.R
• Checks 12 <tspan> elements across 6 panels × 2 variables for both
  facet_grid and facet_wrap
• Currently fails: length(labels) == 0 (no tspans exist yet)

Commit 2 — implementation

• R/z_facets.R: change collapse = "; " / ", " -> collapse = "\n"
• inst/htmljs/animint.js: split on \n, render each line as <tspan>

[codecov]

Codecov Report

❌ Patch coverage is 75.60976% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.01%. Comparing base (9dce861) to head (82f6f0c).

Files with missing lines	Patch %	Lines
inst/htmljs/animint.js	72.72%	9 Missing ⚠️
R/z_facets.R	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   73.02%   73.01%   -0.02%     
==========================================
  Files         164      164              
  Lines        8837     8870      +33     
==========================================
+ Hits         6453     6476      +23     
- Misses       2384     2394      +10     

Flag	Coverage Δ
javascript	81.10% <72.72%> (-0.14%)	⬇️
r	69.15% <87.50%> (+<0.01%)	⬆️

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[tdhock]

thanks, can you please post a screenshot using the new code?