Adapter lowering corrupts string data in option/variant/flavorful types

[avrabe]

Context

Adding wit-bindgen fixtures for variants, options, and flavorful type families (issue #10) revealed that fusion + validation pass, but runtime execution traps due to corrupted string data in cross-component calls.

Reproduction

With fixtures generated from wit-bindgen v0.52.0:

cargo test --package meld-core --test wit_bindgen_runtime test_runtime_wit_bindgen_options

Observed behavior

• options: option<string> roundtrip produces Some("189") instead of Some("foo")
• flavorful: list-in-variant tests produce "\u{1e}\0\0" instead of "foo"
• variants: variant types with string payloads produce corrupted data

The string pointer/length pairs are being read from wrong offsets or wrong memory after adapter lowering.

Root cause

The adapter code path for identifying pointer pairs in function parameters (collect_pointer_positions in parser.rs:1315-1343) explicitly ignores Option, Result, and Variant types:

// Scalars, options, results, resources — no pointer pairs at param level
_ => {}

In the Canonical ABI, option<string> flattens to [i32(discriminant), i32(ptr), i32(len)]. Since the resolver reports no pointer pairs for these types, the adapter does not copy the string data across memories. The callee receives a pointer that references the caller's memory, reading garbage.

Fix needed

collect_pointer_positions and collect_pointer_byte_offsets must recurse into:

• Option<T> — discriminant at offset 0, then payload at offset 1
• Result<T,E> — discriminant at offset 0, then ok/err payload at offset 1
• Variant — discriminant at offset 0, then case payloads at offset 1

The adapter must emit conditional copy logic: only copy the pointer data when the discriminant indicates the variant is active (e.g., Some for option, Ok/Err for result).

This requires:

1. Extending pointer_pair_positions to include option/result/variant payloads
2. Generating conditional if discriminant == X then copy blocks in the adapter
3. Handling the case where different variant arms have different layouts (e.g., result<string, u32>)

Affected code

• meld-core/src/parser.rs: collect_pointer_positions, collect_pointer_byte_offsets
• meld-core/src/adapter/fact.rs: generate_memory_copy_adapter, generate_retptr_adapter
• meld-core/src/resolver.rs: collect_param_copy_layouts, collect_result_copy_layouts

STPA traceability

• SR-16 (recursive inner pointer fixup)
• LS-R-2 (CopyLayout misclassification)
• UCA-A-7 (wrong element stride)

Affected tests

Tests are currently handled with graceful degradation (log + skip):

• test_runtime_wit_bindgen_variants
• test_runtime_wit_bindgen_options
• test_runtime_wit_bindgen_flavorful

Fusion (CoreModule + Component) tests pass for all three. ref #10