chore: Updated SDK to polkadot-stable2512

[Klapeyron]

Description

Bump all Polkadot SDK dependencies from polkadot-stable2509 to polkadot-stable2512-3.

Release notes:

• polkadot-stable2512 (2509 → 2512)
• polkadot-stable2512-3 (patch)

Key upstream breaking changes addressed

• LazyBlock (paritytech/polkadot-sdk#9480): execute_block and check_inherents runtime APIs now take <Block as BlockT>::LazyBlock instead of Block. This defers decoding of block extrinsics until they are actually needed, avoiding unnecessary deserialization overhead. Wire-format is identical, so this is only a type-signature change for downstream runtimes.
• tracing_execute_block (paritytech/polkadot-sdk#9871): spawn_tasks / build_network now requires a tracing_execute_block parameter. This allows parachains to supply custom block execution logic for trace_block RPC (e.g. registering ProofSizeExt). Solochains set it to None.

Runtime migration

pallet_session_validator_management::migrations::v1::LegacyToV1Migration<Runtime> has been moved from a standalone Migrations type alias (passed as the last Executive generic param) into frame_system::Config::SingleBlockMigrations. This follows paritytech/polkadot-sdk#9638 which deprecated the Executive migrations param — the SDK now wants all single-block migration config centralised in frame_system::Config so it's visible in one place alongside multi-block migrations (paritytech/polkadot-sdk#1781). The migration itself is unchanged; only where it is registered has moved.

Changes in this PR

• Cargo.toml / Cargo.lock: Update all ~65 polkadot-sdk workspace dependency tags stable2509 → stable2512-3
• demo/runtime/src/lib.rs: Adapt to breaking SDK changes:
  • execute_block and check_inherents signatures updated for LazyBlock
  • Move LegacyToV1Migration into SingleBlockMigrations config item and remove standalone Migrations type alias + its Executive type param
• demo/node/src/service.rs: Add required tracing_execute_block: None field
• toolkit/block-participation/primitives/src/lib.rs: Remove unused McEpochNumber import
• README.md: Update SDK badge to stable2512
• changelog.md: Note the dependency bump

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff