fix: track conditional reassignments for proper context propagation check

[mpyw]

Summary

• Fixed false negative when a variable is conditionally reassigned with closures that don't all capture context
• Added FuncLitAssignment struct to track whether an assignment occurs inside a control structure (if/for/switch/select)
• Changed logic: now checks ALL reachable assignments (from the last unconditional assignment onwards)
• ALL assignments in the checked range must capture context for the check to pass

Problem

Previously, code like this would not trigger a warning:

func bad(ctx context.Context) {
    fn := func() { _ = ctx }
    if condition {
        fn = func() { fmt.Println("no ctx") }  // Reassigned without context!
    }
    go fn()  // No warning - but should warn!
}

The analyzer only looked at the "last" assignment and would see the context-capturing one from the initial assignment if the reassignment wasn't the last in AST order.

Solution

• Track ALL assignments with conditionality information using inspector.WithStack
• Find the last unconditional assignment as the starting point
• Check all assignments from that point onwards (including conditional ones)
• ALL must capture context for the check to pass

Test plan

• All existing tests pass
• Added 3 new test cases for conditional reassignment patterns
  • badConditionalReassignFirstUsesCtx: First uses ctx, conditional doesn't → should warn
  • badConditionalReassignFirstNoCtx: First doesn't use ctx, conditional does → should warn
  • goodConditionalReassignAllUseCtx: All assignments use ctx → should NOT warn

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.86207% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.96%. Comparing base (4fb020b) to head (5c2a65e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/probe/assignment.go	54.71%	22 Missing and 2 partials ⚠️
internal/checkers/goroutine.go	95.83%	1 Missing and 1 partial ⚠️
internal/probe/capture.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   84.38%   83.96%   -0.43%     
==========================================
  Files          24       24              
  Lines        1710     1808      +98     
==========================================
+ Hits         1443     1518      +75     
- Misses        151      173      +22     
- Partials      116      117       +1     

Flag	Coverage Δ
unittests	83.96% <75.86%> (-0.43%)	⬇️

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