Fix nested selected link joins

[dodok8]

Summary

Fix nested selected single-link shape planning so descendant selected links join from the selected parent alias instead of the root source, while keeping SQL aliases unique when selected link names repeat or collide with root-relative path aliases.

Closes #26

Scope

• Recursively plan selected single-link child shapes in sqlite-query-plan.
• Carry the selected parent alias into nested selected link planning.
• Assign planner-owned SQL aliases when repeated nested selected link names would collide.
• Reserve root-relative path aliases before assigning nested selected link aliases.
• Reuse the selected-shape alias plan across joins, selected values, and result-shape references.
• Keep Many cardinality distinct from nullable parent sources so unsupported multi-link shapes do not get lowered as single-link joins.
• Use LEFT JOIN for descendant selected joins under optional selected sources so root rows are preserved.
• Add planner fixtures and tests for required, optional, repeated-name, root-path-collision, and multi-cardinality nested selected shapes.
• Add query-pipeline execution coverage for Post.author.best_friend and repeated User.best_friend.best_friend nested selection.

Spec and plan alignment

List the spec/ and plan/ files checked for this change. If the change
intentionally diverges from them, explain why and include the required document
update in this pull request.

• spec/query.md: selected relation fields require nested shapes, and the query pipeline must preserve nested result shapes.
• spec/sqlite-query-plan.md: SQLite Plan owns aliases, joins, selected value slots, result-shaping metadata, and inner/left join choices for required and optional links. Multi-link nested shapes are specified for follow-up planning rather than single-link joins.
• plan/sqlite-query-plan-implementation-plan.md: the change stays inside sqlite-query-plan and keeps SQL string rendering in sqlite-query-sqlgen.
• No intentional spec divergence.

Tests

List the automated and manual checks performed.

• cargo test --workspace
• Other: cargo test -p sqlite-query-plan sqlite_select_plan_can_join_nested_selected_single_link
• Other: cargo test -p sqlite-query-plan sqlite_select_plan_uses_left_join_for_nested_selected_link_under_optional_source
• Other: cargo test -p sqlite-query-plan sqlite_select_plan_uses_unique_aliases_for_repeated_nested_selected_link_names
• Other: cargo test -p sqlite-query-plan sqlite_select_plan_avoids_root_path_alias_collision_with_nested_selected_link
• Other: cargo test -p sqlite-query-plan sqlite_select_plan_preserves_multi_link_cardinality_under_optional_source
• Other: cargo test --test select_execution select_pipeline_executes_nested_selected_single_link_shape
• Other: cargo test --test select_execution select_pipeline_executes_repeated_nested_selected_single_link_names

Review notes

Call out API boundaries, temporary rules, migration concerns, or follow-up work
that reviewers should inspect closely.

• The fix is scoped to SQLite planning. The resolver and backend-independent IR contracts are unchanged.
• The query-pipeline fixture still uses raw SQL inserts because Gelite does not yet have an insert pipeline.
• Nested result shaping still uses the current flat selected-value execution path; this PR fixes the join source, join kind, and alias identity needed by that path.
• Multi-link selected shapes remain unsupported in the SQLite select planner until follow-up fetch planning is implemented.

AI assistance

Disclose AI tool usage according to AI_POLICY.md.

• No AI tools were used.
• AI tools were used and are disclosed here and in the relevant commit
  messages with Assisted-by trailers.

Tools and models:

• Codex:gpt-5.5

[dodok8]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

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Codex Review: Didn't find any major issues. 👍

Reviewed commit: bc9ee49488

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