fix: scope duplicate output checks by branch path

[hjotha]

Closes #489.

Summary

• validate IF and enum-split branch bodies with cloned output-variable scopes
• make duplicate-output DESCRIBE warnings track variables per normal sequence-flow path
• keep linear duplicate-output errors/warnings intact while avoiding false positives for mutually exclusive branches

Validation

• make build
• make test
• make lint-go

[ako]

Review: fix: scope duplicate output checks by branch path

Verdict: Approve with minor notes

What the PR does

Two coordinated fixes for false-positive duplicate output variable warnings/errors when the same output variable name is used in mutually exclusive branches (IF/ELSE, enum split cases):

1. validateScopedStatements — clones varTypes and declaredVars before validating branch bodies so cross-branch variable declarations don't bleed into sibling branches.
2. duplicateOutputVariableWarnings rewrite — replaces flat occurrence-counting with a path-following graph traversal that clones seen and path maps at each step, making each execution path independent.

No blockers

Minor issues

O(N²) map cloning in duplicateOutputVariableWarnings
cloneIDBoolMap(path) is called at every node in the graph. For a linear flow of N nodes this creates N maps of sizes 1, 2, ..., N — O(N²) total allocations. For typical microflows (tens of nodes) this is fine, but worth a comment if microflows with hundreds of generated nodes become common.

starts fallback uses all objects
When no StartEvent is found, starts is seeded from all NodesByID values. For a large microflow this triggers O(N) separate DFS traversals from every node, many of which will be immediately terminated by the path guard. Correct and safe, just slightly surprising — a brief comment would help future readers.

Positive notes

• Clean PR: no bundled unrelated changes (unlike several recent PRs on this branch — this is the right scoping)
• Four focused tests that directly reproduce the reported false-positive scenarios
• validateScopedStatements is the minimal correct fix (8 lines, no API surface change)
• Path-scoped traversal correctly models exclusive-branch semantics: a variable assigned in the true branch cannot be "seen again" in the false branch