Fixes multi-state initial configuration

[johnnyt]

Changes initial field from String.t() | nil to [String.t()] to support space-separated initial states per W3C SCXML specification. Adds parsing logic to split "state1 state2" syntax into ["state1", "state2"] list.

Updates interpreter, validator, and test files to handle list format. Fixes parser bug where space-separated states were treated as single IDs.

Test coverage: W3C tests test576 and test413 now pass, all 1028 internal tests pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/statifier/parser/scxml/element_builder.ex	66.66%	1 Missing ⚠️
lib/statifier/validator/initial_state_validator.ex	94.73%	1 Missing ⚠️

Files with missing lines	Coverage Δ
lib/statifier/document.ex	100.00% <ø> (ø)
lib/statifier/interpreter.ex	90.57% <100.00%> (-0.83%)	⬇️
lib/statifier/parser/scxml/element_builder.ex	99.04% <66.66%> (-0.96%)	⬇️
lib/statifier/validator/initial_state_validator.ex	91.48% <94.73%> (-3.52%)	⬇️

... and 1 file with indirect coverage changes

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