refactor: Resolve options in a single pass shared by every insert path

[mhiro2]

Summary

• Options now resolve in one pass shared by InsertOne, InsertMany, Build, and the Builder: traits expand in place, sequence options resolve per record, Ref resolves straight into its relation's option set, and placement rules are checked as each option resolves
• Only under Ref is rejected together with the other root-only options, and the planner no longer carries a dead sequence field or a duplicate Only placement check
• Option precedence is pinned by tests that run the same option lists through every insert path, and documented in the guide, godoc, architecture notes, and skill

Changes

• 7576180 : refactor: resolve options in a single pass shared by every insert path
  • Each option resolves itself into its node's option set, replacing the separate expansion, collection, and validation passes
  • Root-only placement, Seq outside InsertMany, and after-insert callback types are rejected during resolution, wherever the option is written
  • Multiple Refs to the same relation merge into one child set in written order, and the unused planner Seqs field is removed
• 74ba101 : refactor(planner): leave Only placement to option resolution
  • The planner drops its allowOnly flag and nested-Only check, since a nested option set can no longer carry Only
• 5df3cc6 : test: pin option precedence across every insert path
  • Runs the same option lists through InsertOneE, BuildE, InsertManyE, and both Builder insert methods, covering Set/Generate/With stages, traits, merged Refs, Use, and WithContext
  • Checks that each path reports the same placement errors
• 26dbffa : docs: describe the shared option resolution pass and its precedence rules
  • Documents which options replace, merge, or accumulate, the fixed Defaults → Generate → Set → With stages, and the root-only options, including Only

[github-actions]

Code Metrics Report

	main (69d7a67)	#105 (5646616)	+/-
Coverage	86.6%	87.1%	+0.4%
Code to Test Ratio	1:1.0	1:1.0	+0.0
Test Execution Time	1m18s	1m22s	+4s

Details

  |                     | main (69d7a67) | #105 (5646616) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          86.6% |          87.1% | +0.4% |
  |   Files             |             59 |             60 |    +1 |
  |   Lines             |           4781 |           4761 |   -20 |
+ |   Covered           |           4145 |           4147 |    +2 |
+ | Code to Test Ratio  |          1:1.0 |          1:1.0 |  +0.0 |
  |   Code              |          17853 |          17808 |   -45 |
+ |   Test              |          18610 |          18853 |  +243 |
- | Test Execution Time |          1m18s |          1m22s |   +4s |

Code coverage of files in pull request scope (88.5% → 89.7%, patch 97.2%)

Files	Coverage	+/-	Patch Coverage	Status
cmd/seedling-gen/normalized_ir.go	90.9%	+0.1%	-	affected
insert.go	87.5%	+1.2%	100.0%	modified
internal/errx/errors.go	79.3%	+2.6%	-	modified
internal/planner/expand_relations.go	83.0%	-0.2%	-	modified
internal/planner/validate.go	84.0%	-0.4%	100.0%	modified
option.go	94.5%	+4.7%	100.0%	modified
option_resolver.go	95.6%	+95.6%	95.5%	added
plan.go	86.5%	+1.5%	100.0%	modified
registry.go	99.0%	+0.9%	-	affected
result.go	89.1%	+0.5%	-	affected

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Reported by octocov