fix: handle host parameter in call_indirect dispatch (issue #19)

[arnoox]

Summary

Fixes issue #19: indirect calls (call_indirect) were missing the host parameter for WASI/import functions.

The bug involved three related issues:

1. Dispatch args mismatch: All dispatch arms used the same args string, but individual functions might differ in whether they need the host parameter
2. Missing host in dispatcher: Functions using call_indirect to dispatch were not marked as needing a host parameter, even when dispatching to import-calling functions
3. Signature codegen: The function signature generation didn't account for call_indirect when determining if a function needs the host parameter

Fix Details

assembly.rs (Bug 2)

• Update function_calls_imports to also detect CallIndirect instructions when the module has function imports
• This ensures dispatcher functions are marked as needs_host = true

instruction.rs (Bugs 1 & 3)

• Add caller_has_host: bool parameter to generate_instruction_with_info
• Replace the shared dispatch args string with per-arm generation
• Each dispatch arm only passes host when both the target function needs it AND the caller has it in scope

function.rs

• Add has_call_indirect helper to detect call_indirect instructions
• Update signature generation to check for call_indirect with imports
• Thread caller_has_host through instruction generation

Tests

Added regression tests:

• crates/herkos-tests/data/wat/indirect_call_import.wat — WAT module with a dispatcher using call_indirect to reach an import-calling function
• crates/herkos-tests/tests/indirect_call_import.rs — integration test verifying the dispatch works correctly

Verification

cargo test -p herkos-tests -- indirect_call_import
cargo test
cargo clippy --all-targets
cargo fmt --check

All tests pass ✅

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