fix: align numeric placeholder generation with babel

[mogelbrod]

The SWC plugin assigned auto-generated numeric placeholder indices ({0}, {1}, …) differently from the reference Babel/JS macro (@lingui/babel-plugin-lingui-macro). Because message IDs and values keys are derived from these indices, the two toolchains could extract different catalogs for identical source, and thus break translation lookups in projects that mix the JS and SWC tooling.

ph({ name }), {label: value} and bare identifiers carry an explicit name and, in the babel macro, never advance the counter. The SWC plugin derived the next numeric index from values_indexed.len(), which also counted named placeholders, so any unnamed expression following a named one was numbered one too high.

<Trans>
  {ph({ available: 1 })} of <Plural value={zones.length} one="# zone" other="# zones" /> available
</Trans>

	message
Before	{available} of {1, plural, one {# zone} other {# zones}} available
After / in JS	{available} of {0, plural, one {# zone} other {# zones}} available

Fix

Replaced the values_indexed.len()-based index with a dedicated numeric_index counter (next_numeric_index) that is advanced only when an auto-numeric placeholder is actually assigned — never for named placeholders. This mirrors getExpressionIndex.

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.99%. Comparing base (300cf86) to head (e08f777).

Files with missing lines	Patch %	Lines
crates/lingui_macro/src/builder.rs	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
+ Coverage   94.95%   94.99%   +0.03%     
==========================================
  Files          10       10              
  Lines        2597     2637      +40     
==========================================
+ Hits         2466     2505      +39     
- Misses        131      132       +1     

Flag	Coverage Δ
unittests	94.99% <98.14%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/lingui_macro/src/jsx_visitor.rs	89.09% <100.00%> (+0.65%)	⬆️
crates/lingui_macro/src/macro_utils.rs	89.26% <100.00%> (+0.05%)	⬆️
crates/lingui_macro/src/builder.rs	98.73% <97.29%> (-0.23%)	⬇️

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[timofei-iatsenko]

TODO: we need to add the same test cases for the babel plugin, so the implementation will not diverge in a future.

The first fix is looks good to me. The second is bothering me, because it think the original implementation of the named placeholders went wrong, and now fixes on top of this implementation making everethying quite complicated and not straightforward.

Could you may be split this into 2 PRs, and i happily approve fix for the first one, and think a bit for the solution for the second one.

[mogelbrod]

@timofei-iatsenko: I've extracted the second bug fix into #241