feat(examples): balance-tracker example module (BLEU-847)

[brunota20]

Summary

Second canonical SDK example: `modules/examples/balance-tracker/`. Subscribes to blocks, reads `eth_getBalance(addr)` for a configured address list, persists each reading under `balance:{addr}` in local-store, and emits a Warn log when the delta against the prior reading exceeds `change_threshold` wei.

What it demonstrates (complementing price-alert)

Pattern	price-alert	balance-tracker
`chain::request`	`eth_call` via sol! function	raw `eth_getBalance` (no ABI)
`alloy-sol-types`	yes	no — keeps wasm small
State (`local-store`)	none	per-address `balance:{addr}` U256 LE
Diff-against-last-seen	n/a	core of the module

Numbers

• 13 host tests pass
• `cargo clippy --target wasm32-wasip2 -p balance-tracker -- -Dwarnings` clean
• `.wasm` 99 KB optimised (vs price-alert 206 KB — ~half because there's no sol! / alloy-sol-types pulled in)

Module-internal

• `Settings { addresses: Vec, change_threshold: U256 }` parsed once at `init`; stored in `OnceLock`.
• `parse_balance_hex(json)` — strip JSON quotes + `0x` + hex-decode. Handles `"0x"` (zero balance), rejects unquoted / non-hex bodies.
• `parse_addresses(raw)` — comma-separated with whitespace tolerance + empty-segment skipping; rejects empty list.
• `abs_diff` / `parse_u256_le` / `u256_to_le_bytes` — pure utilities.

Stacks on #18 (BLEU-846 price-alert example).

Linear: BLEU-847.

Test plan

• `cargo test -p balance-tracker` — 13 host tests.
• `cargo clippy --target wasm32-wasip2 -p balance-tracker -- -Dwarnings`.
• `cargo clippy -p balance-tracker --tests -- -Dwarnings`.
• `cargo build --target wasm32-wasip2 --release -p balance-tracker` — 99 KB.
• End-to-end against a Sepolia RPC + 2 EOAs — pending the multi-module supervisor.

[linear-code]

BLEU-847