`feat` Existence tests 2/4: scoped inner predicates — forward traversal, nesting, cross-scope guard

[0x054]

Parent

#311 (feat PRD: Existence tests — .exists() on reverse and M2M relations)

What to build

Scoped existence tests: the optional inner lambda, resolved as a full ferro predicate over the child model through the existing predicate resolver.

Everything a root predicate can do works inside the test: every operator, forward-FK traversal (rendered as joins inside the EXISTS subquery under unchanged ADR-0006 semantics — INNER at every hop, narrowing), &/|/~ composition, and nested existence tests to arbitrary depth (the exists node's inner tree is an ordinary condition tree, recursion for free).

Cross-scope references are rejected in this slice, the moment inner lambdas exist: an inner-tree leaf built from any proxy other than the inner lambda's own parameter, or a field proxy appearing as a comparison right-hand side, is a build-time error pointing at the deferred capability (#309). Silent misrendering is the failure mode this guard exists to prevent.

Demo: the full #308 query —

Txn.where(lambda t: t.category_id.in_(ids)
                  | t.lines.exists(lambda l: l.category_id.in_(ids)))

— a three-line split matches once, child-less roots survive the OR through the root branch, and keyset order_by + limit compose unchanged, on both backends.

Acceptance criteria

• Inner lambda supports every operator and &/|/~ over the child model
• Forward traversal inside the subquery renders joins within the EXISTS under ADR-0006 semantics (covered e2e)
• Nested exists-in-exists works; depth 2 pinned e2e and by a golden vector
• The feat OR-composition across a left-joined reverse child table (PRD from Pinch) #308 shape passes e2e on both backends: multi-line match → one root row; child-less root survives the OR; keyset composition
• Explicit grouping semantics covered: t.lines.exists(lambda l: A & B) (one child matches both) vs t.lines.exists(A-test) & t.lines.exists(B-test) (some child each) return different, correct row sets
• Cross-scope reference and field-proxy-as-RHS raise build-time errors naming feat Cross-scope correlation in existence tests (deferred from ADR-0007) #309 (messages pinned by tests)
• Hand-authored golden vectors: scoped 1-hop exists, nested exists

Blocked by

• feat Existence tests 1/4: bare .exists() on reverse FK — the tracer bullet #314