feat(examples): price-alert Chainlink oracle reader (BLEU-846)

[brunota20]

Summary

First canonical SDK example: `modules/examples/price-alert/`. Polls a Chainlink AggregatorV3 price oracle on every block (throttled by `every_n_blocks`) and emits a Warn-level log when the answer crosses a config-supplied threshold.

What it demonstrates

Pattern	Implementation
`chain::request` + ABI decode	`sol! interface AggregatorV3 { latestRoundData(...) }` with `abi_decode_returns`
shepherd-sdk helpers	`chain::eth_call_params` + `chain::parse_eth_call_result`
Config-driven module	`OnceLock` initialised in `init`; read on every event

Module-internal pieces

• `Settings` (renamed from `Config` to avoid clashing with the wit-bindgen `Config` type alias for the `init` arg).
• `Direction { Above, Below }` + `classify(answer, threshold, direction)` — pure helper, 2 edge tests at the threshold boundary.
• `scale_threshold(decimal, decimals)` — hand-rolled because alloy lacks a `Decimal::parse_units` helper. Handles sign, missing decimal point, short/long fractional, rejects garbage. 5 unit tests.
• `parse_config(entries)` returns `Result<Settings, String>` with human-readable errors. 4 unit tests (happy path, defaults, unknown direction, missing key).

module.toml

`capabilities = ["logging", "chain"]` (no local-store, no cow-api). `[[subscription]]` block on Sepolia. `[config]` defaults to the canonical Sepolia ETH/USD feed (`0x694AA1...5306`) with a 2500.00 USD threshold + "below" direction.

Numbers

• 11 host unit tests pass
• clippy clean on host + wasm32-wasip2
• `.wasm` 206 KB optimised (vs M2 modules at 275–305 KB — smaller because no cow-api / cowprotocol surface)

Stacks on #16 (BLEU-844 SDK docs).

Linear: BLEU-846.

Test plan

• `cargo test -p price-alert` — 11 host tests.
• `cargo clippy --target wasm32-wasip2 -p price-alert -- -Dwarnings`.
• `cargo clippy -p price-alert --tests -- -Dwarnings`.
• `cargo build --target wasm32-wasip2 --release -p price-alert` — 206 KB.
• End-to-end against a Sepolia RPC + Chainlink ETH/USD feed — pending the multi-module supervisor (PR feat(ethflow-watcher): decode CoWSwapEthFlow OrderPlacement (BLEU-832) #9 upstream).

[linear-code]

BLEU-846