fix(flow): ignore transition labels in flow.mmd

[tupe12334]

Fixes #232

Problem

A flow.mmd with a transition label (a --> b : text) failed to load with state 'a' has no path to [*]. parse_mmd split the line on --> and stored b : text as the target state. Transition labels are valid Mermaid stateDiagram-v2 syntax.

Change

• core/src/flow.rs: split_transition drops the : label suffix from the transition target.
• core/src/flow_tests.rs: new ignores_transition_labels test (labeled branch + labeled terminal transition).
• CHANGELOG.md: entry under Unreleased.
• core/Cargo.toml: deny clippy::explicit_into_iter_loop. The repo's pre-push checklist requires one lint ratchet per push. This lint has no violations, so the change is one line.

Verification

• cargo test, cargo clippy --all-targets -- -D warnings, cargo fmt --check and cargo doc with -D warnings pass.
• End to end with the built binary: a branching flow with tests --> docs : yes / tests --> fix : no now validates. The hook blocks on each state, follows the fix branch on ack.sh fix, and approves after the last ack. Published steplock 0.2.2 rejects the same flow.

Opened by the moadim routine "Nightly lib dogfood → issue + PR (polyhook, block-no-verify, steplock)".

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