fix(verify): diagnostic for x=+x 1 mutation pattern in @ loops (ILO-450)

[danieljohnmorris]

Summary

• Adds ILO-T047 warning when a Stmt::Let inside a @binding loop rebinds the loop's own iterator variable (e.g. x=+x 1 inside @x xs or @x 0..10)
• The loop engine resets the iterator from its own cursor each iteration, so the rebind is silently discarded — classic wrong-output footgun
• Hint suggests the correct accumulator pattern (acc=0;@x ...{acc=+acc x}) or fold
• Nested loops: only the innermost binding triggers the warning (outer-var rebind inside an inner loop is not flagged)

Test plan

• loop_var_rebind_foreach_warns — fires on @x xs{x=+x 1}
• loop_var_rebind_forrange_warns — fires on @i 0..10{i=+i 1}
• loop_var_rebind_other_binding_no_warn — accumulator pattern acc=+acc x is clean
• loop_var_rebind_nested_inner_warns — inner loop var warned, not outer
• loop_var_rebind_outer_in_nested_no_warn — outer var rebind inside inner loop not flagged
• All 3499 existing unit tests pass

Closes ILO-450

🤖 Generated with Claude Code

[danieljohnmorris]

mini pc is reviewing this

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
3808	1	3807	0

View the top 1 failed test(s) by shortest run time

ilo::coverage_interpreter::mapr_short_circuits_on_err

Stack Traces | 0.014s run time

thread 'mapr_short_circuits_on_err' (42149) panicked at tests/coverage_interpreter.rs:950:5:
stderr={"code":"ILO-P024","labels":[{"col":49,"end":52,"line":1,"message":"here","primary":true,"start":48}],"message":"fn declarations are top-level only; this one is inside another function's body","notes":[],"severity":"error","suggestion":"use an inline lambda for a one-off helper that captures locals (e.g. `proc = (x:n>n; +x rows)` or `proc = {x> +x rows}`), or lift the helper to the top level and pass the captured value as an explicit parameter"}
{"code":"ILO-T005","labels":[{"col":64,"end":79,"line":1,"message":"","primary":true,"start":63}],"message":"undefined function 'f': its definition failed to parse","notes":["in function 'main'"],"severity":"error","suggestion":"fix the parse error first (see ILO-P024 reported earlier in this file); other references to 'f' are suppressed until then"}

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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