Add Rust ABI support for WASM subgraphs

[cargopete]

Summary

Adds support for Rust-compiled WASM subgraphs as an alternative to AssemblyScript. This introduces a new rust_abi/ module parallel to the existing asc_abi/, with a clean ptr+len FFI protocol and TLV entity serialization.

• rust_abi/ module (~1,450 LOC) — ToRustWasm/FromRustWasm traits, TLV entity serialization, trigger serialization (RustLogTrigger/RustCallTrigger/RustBlockTrigger), and all host function linker wrappers (store, crypto, logging, data sources, IPFS, ethereum calls) with async + gas metering
• Language detection — is_rust_module() detects "graphite" namespace imports; MappingLanguage enum stored on ValidModule
• Dispatch — build_linker() branches on language; handle_trigger_rust() uses Rust calling convention (allocate → write bytes → call handler(ptr, len) → reset_arena)
• Compatibility — skips parity_wasm gas injection for Rust modules (parity_wasm can't parse modern WASM features like bulk-memory); skips AS-specific exports (id_of_type, _start)
• Ethereum triggers — ToRustBytes impl for all 3 trigger types (Log, Call, Block)
• 14 unit tests passing for entity roundtrips, trigger serialization, type roundtrips

Live-tested

Deployed an ERC20 subgraph (compiled with Graphite SDK) to this fork, indexing real USDC Transfer events from Ethereum mainnet. Full pipeline verified: block ingestion → event scanning → Rust WASM handler → entity storage → correct GraphQL query results.

Architecture

                    ┌─────────────────────────┐
                    │    HostExports<C>       │  ← Language-agnostic (unchanged)
                    │  (store, crypto, ipfs)  │
                    └────────────┬────────────┘
                                 │
              ┌──────────────────┼──────────────────┐
              │                                     │
    ┌─────────┴─────────┐             ┌─────────────┴────────────┐
    │  AscAbiHost       │             │  RustAbiHost             │
    │  (existing code)  │             │  (new, this PR)          │
    │  AscPtr<T>        │             │  ptr+len, TLV serde      │
    └───────────────────┘             └──────────────────────────┘

Known gaps (not blocking review)

• Gas metering — parity_wasm gas injection is bypassed for Rust modules. Needs wasmtime fuel metering or a WASM-2.0-compatible alternative.
• NEAR triggers — stubbed as unimplemented! (Ethereum-only for now)
• Offchain/subgraph triggers — stubbed as empty bytes

Test plan

• Existing graph-node tests pass (14 rust_abi + 17 NEAR chain tests confirmed locally)
• WASM integration test: load Rust WASM, serialize trigger, call handler, verify entity
• Live test: deploy Rust subgraph to running fork, index mainnet events, query via GraphQL
• Verify AS subgraphs are completely unaffected (no changes to AS code paths)