fix(opt): wasm_to_ir missing result vreg for WasmOp::Call

[avrabe]

Closes the last latent unmapped-vreg gap surfaced by the systematic AAPCS audit (PR #108) when compiling examples/wat/simple_add.wat — the recursive fib function caused synth compile --cortex-m to panic on vreg v13 (the second Call result, consumed by the outer i32.add).

Root cause

optimizer_bridge::wasm_to_ir's match on WasmOp had no arm for WasmOp::Call(idx), so it fell through to Opcode::Nop. The IR therefore had no record that inst_id N defined a vreg; any downstream consumer (here, the outer i32.add) resolved its src via get_arm_reg → unmapped → defensive panic (PR #101) or, under the silent R0 fallback that's in main today, silently consumed whatever value happened to be in R0.

Fix

• New Opcode::Call { dest, func_idx } in synth-opt
• WasmOp::Call → Opcode::Call in wasm_to_ir
• Opcode::Call → ArmOp::Bl { label: "func_<idx>" } in ir_to_arm, with dest → R0 registered in vreg_to_arm (AAPCS: R0 holds the return value)

Scope (narrow)

This is the "compile cleanly, don't panic on lawful WASM containing call" fix. The IR still does NOT:

• Model call-clobber-R0..R3 invalidation of surviving vregs across the BL
• Pop arg slots from the value-tracking stack

Both are deferred to a broader call-boundary regalloc rework.

Tests

• fib_compiles_through_optimized_path — full fib WAT through optimize_full + ir_to_arm; asserts two BL func_1 are emitted plus the outer i32.add lowering
• call_emits_bl_in_optimized_path — minimal standalone WasmOp::Call → BL func_5 round-trip

A third end-to-end test through ArmBackend was removed during local review because synth-synthesis sits below synth-backend in the dep graph (would close a cycle). The existing synth-cli/tests/wast_compile.rs test suite already exercises the full pipeline.

Verification

• cargo test --workspace — all pass
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• synth compile examples/wat/simple_add.wat --cortex-m -o /tmp/sa.elf — succeeds (no panic)

Unblocks

PR #101 (defensive panic) — once this lands, the defensive panic can ship as a permanent guard against the wasm_to_ir-gap class.

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