Implement nested repeat directive (#8)

[piotrszul]

Summary

Closes #8.

Implements the repeat directive with no positional constraints beyond the spec's per-select-node mutual exclusion of forEach / forEachOrNull / repeat. Wherever a select node is structurally legal, a select node carrying repeat is now handled correctly by the query generator.

Changes

• Tree-walker query generator — replaces the processor-based architecture (ForEachProcessor, RepeatProcessor, SelectClauseBuilder, SelectCombinationExpander) with a recursive tree-walker in src/queryGenerator/treeWalker/. Context (current JSON source, partition keys, accumulated APPLY chain) threads through descent, eliminating the previous post-hoc rewiring step that couldn't represent complex nesting relationships.
• Submodule bump — sqlonfhir advances to aehrc/sql-on-fhir-v2:repeat-updates (commit 27ee2c6), which adds twelve test cases covering the full structural surface of the operator: linear nestings, sibling combinations, triple-nested mixed combinations, and operator-pairing coverage (unionAll inside repeat, multi-path repeat inside forEach, etc.). The submodule URL is repointed to the aehrc fork pending the upstream PR to FHIR/sql-on-fhir-v2.

Test plan

• All 19 tests in sqlonfhir/tests/repeat.json pass against the MSSQL implementation.
• All 19 tests pass against the sof-js reference implementation.
• Open upstream PR on FHIR/sql-on-fhir-v2 for the new test cases.

[piotrszul]

Code review

Found 1 issue:

1. Context.nullable is tracked but never consumed in repeat.ts — repeat nodes nested under forEachOrNull always use INNER JOIN to the CTE, which will drop the null rows that OUTER APPLY from forEachOrNull preserves. The nullable field exists on Context precisely to drive this join-type decision, and it is correctly propagated in forEach.ts (line 81), but buildJoinClause in repeat.ts hardcodes INNER JOIN regardless of ctx.nullable.

sof-mssql/src/queryGenerator/treeWalker/operators/repeat.ts

Lines 66 to 73 in 5bd6f92

	}
	/**
	* Outer-SELECT join condition aligning this CTE's rows with the enclosing
	* partition (resource id plus any forEach/repeat keys above this scope).
	*/
	function buildJoinClause(cteAlias: string, ctx: Context): string {
	const joinConditions = ctx.partitionKeys

See also the declaration in types.ts:

sof-mssql/src/queryGenerator/treeWalker/types.ts

Lines 51 to 54 in 5bd6f92

	resourceAlias: string;
	/** Current JSON source expression, e.g. "r.json", "forEach_0.value". */
	source: string;
	/** Ordered, monotonically appended as the walker descends. */

🤖 Generated with Claude Code

_{- If this code review was useful, please react with 👍. Otherwise, react with 👎.}

[johngrimes]

Thanks @piotrszul!

I noticed a few minor issues.