Initialize default traversal state when component enumerators run

[OskarEichler]

Summary

Create default traversal state when a component-from-node enumerator runs, so it can be enumerated again or rewound. Preserve explicitly supplied state objects for callers that intentionally share traversal state.

Reproduction

graph = {1 => [2], 2 => [3], 3 => []}
children = ->(node, &block) { graph.fetch(node).each(&block) }
enum = TSort.each_strongly_connected_component_from(1, children)
p enum.to_a # [[3], [2], [1]]
p enum.to_a # currently [[1]]
enum.rewind
p enum.to_a # currently [[1]]

The default hash and stack are currently created before to_enum and captured in its arguments. Later enumerations reuse visited-node state. Deferring allocation until the block path makes implicit state fresh on each run. The mixin entry point gets the same correction.

Verification

Ruby 4.0.6 through rbenv:

• Existing suite: 8 tests / 17 assertions, no failures.
• 15 external checks pass for class/mixin entry points, repeated to_a, next then rewind, take then complete enumeration, and explicit shared state.
• Combined with the separate direct-block-forwarding patch on a 0.2.0 release-based branch: the same suite, these checks, and 4,000 graph checks pass; graph results match the baseline.
• Ruby syntax, gem build and git diff --check pass. No dependency or version changes.

No tests were added/modified under the requesting project's policy; repros are external. Local verification covers macOS/Ruby 4.0.6, not the full upstream matrix. Explicitly supplied state remains caller-owned and can intentionally affect subsequent enumerations.

Breaking-change notes

Repeated enumeration with implicit state now repeats the complete traversal instead of silently omitting previously visited children. Explicit state behavior is preserved. Passing nil for either optional state argument now selects a fresh default instead of failing incidentally. Cycle/self-loop semantics and traversal ordering are unchanged.