fix(splat): stop char-spreading leftover primitive metas into info

[abhu85]

Summary

format.splat() merges any splat arguments beyond the interpolation tokens into info via Object.assign. When a leftover argument is a primitive string, Object.assign spreads its characters into index-keyed properties.

Fixes #109

Problem

logger.error('one %s', 'two', 'three');
// { "0":"t","1":"h","2":"r","3":"e","4":"e", level:"error", message:"one two" }

After %s consumes 'two', the leftover 'three' is a string, and Object.assign(info, 'three') treats it like an object with enumerable index keys, polluting info with {0:'t',1:'h',...}.

Solution

In Splatter._splat, only merge a leftover meta into info when it is a non-null object (typeof meta === 'object' && meta !== null). Object and array metas are unchanged; primitive leftovers are no longer char-spread.

Test Plan

• Added a regression test asserting input 'one %s' + [SPLAT] = ['two','three'] yields message === 'one two' with no 0..4 index properties on info.
• Fails before the fix (info[0] === 't'), passes after.
• Full suite: 170 passing, 0 failing. Lint: 0 errors.

Compatibility

Object/array metas (the documented use case) behave exactly as before — covered by existing passing tests. Only the spurious character-index pollution from primitive leftovers is removed. The separate no-token code path is intentionally left untouched to avoid contradicting its existing test.