wasm-encoder: add ComponentBuilder::start()

[zacharywhitley]

Summary

Adds a public ComponentBuilder::start() helper for emitting component start sections. Currently every other section shape has a builder helper (import, export, alias, lift_func, lower_func, core_module, core_module_raw, component_raw, instantiate, instantiate_exports per #2655, raw_custom_section, …), but component start is missing — callers have to fall back to the lower-level Component::section(&ComponentStartSection { … }) interface and manually maintain their own value-index counter.

pub fn start(&mut self, function_index: u32, args: Vec<u32>, results: u32) -> u32 {
    self.flush();
    self.component.section(&ComponentStartSection {
        function_index,
        args,
        results,
    });
    let base = self.values.count;
    for _ in 0..results {
        self.values.add(None);
    }
    base
}

Returns the index of the first produced value in the component value index space; subsequent produced values occupy contiguous indices from first+1 up to first+results-1.

Motivation

Downstream tooling (specifically wasmos-component-opt, a component-level optimizer) needs to round-trip components that carry start sections. The reader / IR / DCE walker are all in place; only the encoding step was gated on this API. The component_start_section_roundtrips integration test in that crate exercises the full path (parse → optimize → re-encode → validate).

Design notes

• Component start is a singleton section rather than an aggregating one, so it isn't routed through the section_accessors macro that handles component_instances, types, imports, etc. The helper flush()es any prior section first so section ordering stays coherent.
• The Component Model spec permits at most one start section per component. This helper does not enforce that — matching the underlying ComponentStartSection encoder's posture; the caller carries the responsibility.
• Return value is deliberately consistent with instantiate_exports (wasm-encoder: add ComponentBuilder::instantiate_exports #2655): a u32 naming the first new index-space entry produced. Callers can iterate first..first+results if they need each slot separately.

Test plan

• cargo build -p wasm-encoder clean.
• cargo test -p wasm-encoder — 36 tests, all pass.
• Downstream integration verified against wasmos-component-opt's component_start_section_roundtrips test (parse a component with a start section → run the -O2 pipeline → re-encode via b.start(...) → validate).