feat: carry the PostageStamp batch events from the wire to the domain

[mfw78]

What does this PR do?

Carry the PostageStamp batch events (BatchCreated, BatchTopUp, BatchDepthIncrease) from the contract wire into the domain BatchEvent, and give the issuer the translation.

Changes

• crates/contracts: sol! declares the three events on IPostageStamp, with field names, order and types matching ethersphere/storage-incentives (master c8686806) and the IPostageStamp events pinned in go-storage-incentives-abi v0.9.4. Two vector tests guard the topic hashes and the decoder field shapes.
• crates/postage: BatchEvent carries the normalised balance (new_value) for TopUp and DepthIncrease and the mined-in block for DepthIncrease; each variant documents where its fields come from and which event body fields are intentionally not carried (totalAmount, topupAmount).
• crates/postage-issuer: a new std-only translation module (created, top_up, depth_increase, expired plus the permanent EventError) converts the decoded logs to BatchEvent, and the crate takes an optional nectar-contracts dependency under std.

Breaking changes

The domain BatchEvent variants change shape (struct variants, new new_value and block fields). nectar is pre-1.0 and the variants were internal to this tree.

Testing

• Unit tests pass: 283/283 across nectar-postage, nectar-contracts and nectar-postage-issuer (nextest)
• clippy --all-targets clean for the default, parallel and sign-parallel feature shapes, fmt clean, doctests clean
• cargo check --locked --no-default-features builds on riscv64imac-unknown-none-elf and wasm32-unknown-unknown (the ci no_std targets) with no new warnings
• Manual testing completed
• Documentation updated (if needed)

Related issues

AI assistance disclosure

AI Assistance: opencode (qwen3.8) used for the implementation and verification.

Notes for reviewers

• Placement: the domain shape stays in nectar-postage (no_std), and the translation from the decoded logs lives in nectar-postage-issuer (std only, where the nectar-contracts generated events are already the consumer).
• Vector provenance: upstream publishes no topic or log vectors for these events, so the vectors are generated. The declarations were checked against both the .sol source (master c8686806) and the pinned v0.9.4 ABI, and the headers in the tests state that.

Checklist

• Code follows project style
• Self-review completed
• Tests added/updated
• No console.logs or debug code left behind
• PR title is descriptive

Closes

• Closes postage, contracts: generate the batch events with sol! instead of hand-writing them #791