Fix #18: emit parse_path in function-call form for lambda params

[piotrszul]

Problem

The default (struct) backend generated DuckDB SQL that fails to bind on DuckDB 1.4.x for any forEach/forEachOrNull select containing a getReferenceKey() column (e.g. EncounterFlat's participant/location):

Binder Error: Referenced column "el" not found in FROM clause!

The same SQL ran fine on DuckDB 1.5.x. Reported in #18.

Root cause

getReferenceKey(Type) expands to reference.where(_splitPath(-2) = 'Type')._splitPath(-1). The _splitPath step inside the where() predicate compiled to el.parse_path('/')[...] — a method call with the lambda parameter as the receiver. When that predicate sits in a list_filter nested inside the forEach's list_transform, DuckDB 1.4.x's binder fails to resolve the lambda parameter.

The issue originally hypothesised parameter shadowing (inner el shadowing outer el). Investigation against DuckDB 1.4.1 showed that is not the cause:

• Renaming the inner parameter to a unique name (el_1) does not fix it.
• Reusing the same name el together with the function-call form works fine.
• The trigger is specifically a lambda parameter used as a method-call receiver (el.parse_path(...)) inside a list_filter predicate nested in another lambda. Nested list_transform with the same construct binds fine; plain (non-receiver) uses of the parameter bind fine.

Fix

Emit the equivalent function-call form parse_path(el, '/')[...] instead of the method-call form el.parse_path('/')[...] when _splitPath operates on a lambda parameter. This binds correctly on both 1.4.x and 1.5.x. The non-lambda _splitPath case stays method-chained so flattenSql can still join it onto the preceding navigation segment.

One-case, general change in src/ddb-sql-builder.js.

Verification

• New e2e test (tests/e2e.test.js) reproduces the issue's minimal case (forEachOrNull: participant + individual.getReferenceKey(Practitioner)) and executes the generated SQL against the bundled DuckDB 1.4.1 — RED before the fix (Binder Error), GREEN after.
• Full suite: 192 pass / 0 fail (baseline 191; +1 new test).
• End-to-end: built the issue's MinimalRepro.json via the CLI and ran the generated .sql on DuckDB 1.4.1 → id,practitioner_id / enc1,prac1.

Notes / out of scope

• This is a DuckDB-1.4.x binder limitation; the generated SQL was already valid on 1.5.x. The fix makes flatquack emit SQL that binds on both.
• The experimental staged backend was already unaffected (it uses UNNEST stages rather than nested list-lambdas), so no change there.
• Branch is based directly on origin/master; no other fixes folded in.

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