fix(planner): Ignore has-many back-references in cycle detection

[mhiro2]

Summary

• Stop rejecting finite graphs that loop through a has-many parent, such as assignment -> employee -> department -> employees, as dependency cycles
• Keep reporting genuinely infinite required recursion, including self and mutual has-many chains
• Document how back-referenced has-many children are treated during cycle detection

Changes

• 172f8e7 : fix(planner): ignore has-many back-references in cycle detection
  • Cycle detection now also compares which relations a frame resolves through a structural back-reference, so a generated has-many child bound to the parent that created it no longer matches the unbound frame that started the chain
  • Bound relations are compared by name rather than by node, so recursion that rebinds a fresh parent on every level is still reported as ErrCycleDetected
  • Planner and public API tests cover the finite back-reference graph and the required self and mutual has-many cycles
• 8ead5bb : docs: describe back-reference handling in cycle detection
  • ARCHITECTURE.md and the guide explain that a has-many child bound back to its creating parent is not treated as a revisit

[github-actions]

Code Metrics Report

	main (4880ed2)	#100 (7d6472e)	+/-
Coverage	86.7%	86.8%	+0.0%
Code to Test Ratio	1:0.9	1:0.9	+0.0
Test Execution Time	48s	1m17s	+29s

Details

  |                     | main (4880ed2) | #100 (7d6472e) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          86.7% |          86.8% | +0.0% |
  |   Files             |             59 |             59 |     0 |
  |   Lines             |           4771 |           4772 |    +1 |
+ |   Covered           |           4139 |           4143 |    +4 |
+ | Code to Test Ratio  |          1:0.9 |          1:0.9 |  +0.0 |
  |   Code              |          17736 |          17740 |    +4 |
+ |   Test              |          17596 |          17715 |  +119 |
- | Test Execution Time |            48s |          1m17s |  +29s |

Code coverage of files in pull request scope (83.2% → 84.6%, patch 100.0%)

Files	Coverage	+/-	Patch Coverage	Status
internal/planner/expand.go	92.8%	+0.2%	100.0%	modified
internal/planner/expand_relations.go	83.5%	+1.5%	-	affected

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Reported by octocov