solidity: write struct fields directly into result memory on deserialize

[deuszx]

Summary

bcs_deserialize_offset_<Struct> previously held every decoded field as a stack local until the final Struct(field0, field1, ...) constructor. At ~2 stack slots per field (the value plus the second slot of the (new_pos, value) tuple return) Yul ran out of the 16-slot budget when these helpers were inlined into wider call graphs. The current linera-bridge workaround is a build.rs shim that rewrites every assembly { ... } block in the generated file to assembly ("memory-safe") { ... } so Yul's optimizer is allowed to spill across them.

Switch the generated code to declare the return value with a named binding (returns (uint256, <Struct> memory result)) and write each field through result.<field> as it is decoded. The only persistent locals are now pos, new_pos, and the result pointer — stack footprint is O(1) regardless of field count, and the final Struct(...) constructor call (which previously memcpy'd N locals into a freshly allocated struct) disappears.

Validation against linera-protocol#6294:

• Before this change, with the in-tree codegen output and the memory-safe annotations stripped, forge build on linera-bridge/src/solidity fails with "Cannot swap Variable var_input_2223_mpos … too deep in the stack by 1 slots" inside the inlined verifyCertificate deserializer chain.
• After regenerating BridgeTypes.sol from the same registry snapshot using this commit's codegen — still without any memory-safe annotations — forge build succeeds. The mark_assembly_memory_safe shim in linera-bridge/build.rs becomes obsolete.

Measured on linera-bridge after regen (via_ir, optimizer_runs=1, solc 0.8.30, evm_version=cancun):

Metric	Old + shim	New (no shim)	Delta
LightClient runtime bytecode	23 296 B	22 725 B	-571 B
EIP-170 headroom for LightClient	1 280 B	1 851 B	+571 B
LightClient deployment size	24 785 B	24 214 B	-571 B
FungibleBridge runtime bytecode	10 987 B	10 958 B	-29 B
DeployLightClient script gas	10 838 349	10 600 945	-237 K

Synthetic gas/bytecode measurement on a 16x Inner struct (Inner = { String, u64, u128, Vec }, via_ir = true, optimizer_runs = 200, solc 0.8.33):

• deserialize gas: 217 224 -> 210 076 (-7 148, -3.3 %)
• harness bytecode: 3 088 B -> 2 759 B (-329 B, -11 %)
• deployment gas: 715 222 -> 644 074 (-71 K, -10 %)

Test Plan

New test test_wide_struct_deserialize_round_trip was added.