[ty] Propagate deletions through nested loop headers

[carljm]

An inner loop can delete a variable on one iteration and then exit through break on a later iteration. The variable can remain unbound after the inner loop and on subsequent outer iterations, but ty did not propagate deletions carried by nested loop headers into the enclosing loop's boundness analysis.

Include nested headers' deletion reachability when computing an enclosing header's reachability. Keep the loop-back bindings intact for type inference, and memoize the structural traversal used during Salsa cycle initialization so shared descendant headers are only visited once.

Related to #28009.

Ecosystem

The ecosystem run reports no stable diagnostic changes. Typing conformance and memory usage are unchanged.

Test plan

• Add while and for mdtests for deletions that survive an inner-loop iteration and a later break, including an explicit continue and a for-else that returns.
• Verify that reassignment after the inner loop restores boundness on subsequent outer iterations.
• Verify that statically unreachable deletions preserve boundness and literal types through several nested loops.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 97.69%. The percentage of expected errors that received a diagnostic held steady at 93.71%. The number of fully passing files held steady at 110/136.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Large timing changes:

Project	Old Time	New Time	Change
dd-trace-py	3.08s	0.92s	-70%

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)