fix(manifest): terminate reachability on recursive sums via List/tuple

[nelsonduarte]

Problem

_contains_fun_via_structs in capa/manifest/_reachability.py (the manifest reachability walker) propagated its cycle-guard (the _seen visited-type set) only when recursing into named struct fields and named sum-variant payloads, but reset it to empty when recursing into type arguments (List<Self>) and tuple elements.

A recursive sum type whose self-reference passes through List<Self> or a tuple, for example

type Condition =
    PathExists(String, Bool)
    AllOf(List<Condition>)
    AnyOf(List<Condition>)
    NotMatch(Condition)

therefore looped forever and raised RecursionError. Because build_manifest -> compute_reachability runs for both --manifest and the Wasm backend (compile_wat), both crashed on such a program; --check was unaffected (it does not build the manifest). Raising the recursion limit does not help: the cycle is infinite.

Discovered while dogfooding --wasi on the downstream policy-eval program (AllOf(List<Condition>)).

Fix

Forward the current visited-set (_seen) through every recursive branch, including the tuple-element and type-argument branches, so the traversal terminates the first time it revisits a type name. The struct-field and sum-payload branches already forwarded it and are unchanged.

The fix only adds termination: the fun-bearing / capability result for every type that already terminated is identical, so no existing manifest or SBOM result changes. The sibling walker _caps_via_type needs no change (it does not expand named structs/sums, so it cannot cycle).

Validation

• Downstream policy-eval now both generates a manifest (python -m capa --manifest) and compiles to Wasm (python -m capa --wasm --transpile policy_eval.capa), both exit 0. (policy-eval itself was not modified.)
• Regression tests added in tests/test_manifest.py (TestRecursiveTypeReachability): recursive sum via List<Self>, via tuple, and directly self-referential all terminate and yield the correct (empty, not-unprovable) capability result; plus an end-to-end build_manifest case on the AllOf(List<Condition>) shape.
• Manifest, IFC, and full suites pass; no existing manifest/SBOM result changed.