feat(macros): make manifest topic drift a compile-time failure

[mfw78]

What

Makes the #[nexum_sdk::module] macro fail the build when a module's
module.toml event_signature does not match the topic-0 of the event
type named in the attribute.

Parity checking is opt-in per module and names both sides (manifest hash
and computed SIGNATURE_HASH) when they drift, so a mismatch is caught
mechanically instead of by a per-module hand-written test.

Adds a topic-parity fixture crate under modules/fixtures/ and wires it
into the CI wasm build matrix and the local justfile build/ci targets.

Why

The on-chain topic a module subscribes to was stated twice: once in code
via sol!/SIGNATURE_HASH, and once in module.toml as event_signature,
which is what the host actually filters on. Nothing kept the two in step.

ethflow-watcher had no guard at all, so a drift there would be silent:
the host subscribes to one topic, the guest decodes another, and the
module never sees the event. twap-monitor's hand-written parity test was
itself found to be a substring match that would have passed while the real
event_signature had drifted.

This closes the gap by construction at compile time rather than relying on
per-module tests, while keeping module.toml as the operator-auditable
admission surface.

Closes #49

Testing

• cargo nextest run --workspace --all-features --no-fail-fast
• cargo test --doc
• CI wasm build matrix extended to build the new topic-parity fixture
  (cargo build --release --target wasm32-wasip2 -p topic-parity, alongside
  the other fixture crates)
• justfile build-fixtures and ci targets updated to include
  -p topic-parity so the fixture is exercised locally the same way as CI

AI Assistance

Implementation by Claude (Fable 5), red-team review by Claude Opus 5, PR by Claude Sonnet 5.