feat(contracts): inspector bond + slashing-module integration

[joannamach250-collab]

Summary

Implements the per-inspector bond + cross-contract slashing flow from #925 alongside the existing collateralised-bond and validator-evidence-slashing models. The existing public methods are not touched — the new functionality is purely additive — so nothing else in the workspace breaks.

contracts/bond_collateral

• set_slashing_module(admin, slashing_module) and set_operator(admin, operator) for one-time wiring.
• deposit_bond(inspector, amount) / withdraw_bond(inspector, amount) / get_bond(inspector) / get_locks(inspector).
• lock_bond(operator, inspector, inspection_id) — operator-gated; any active lock blocks withdraw_bond with BondLocked.
• unlock_bond(operator, inspector, inspection_id).
• execute_slash(admin, inspector, slash_amount, inspection_id, reason) — admin-gated; cross-contract call to slashing_module.slash to record the slash, then reduces the inspector's bond by exactly slash_amount. Fails fast with InsufficientBond when slash_amount > bond.
• New error variants: BondLocked, InsufficientBond, LockAlreadyExists, LockNotFound, SlashingModuleNotSet.

contracts/slashing_module

• set_bond_contract(admin, bond_contract) / bond_contract() — registers the single bond contract authorised to call slash.
• slash(caller, inspector, amount, inspection_id, reason) -> i128 — appends an InspectorSlashRecord { inspection_id, amount, reason, slashed_at } to the inspector's history. Rejects callers that aren't the registered bond contract with NotAuthorized; rejects amount <= 0 with ArithmeticError.
• get_slash_history(inspector) -> Vec<InspectorSlashRecord>.

Both crates now expose rlib in addition to cdylib so tests can cross-import the generated *Client types.

Acceptance criteria

• withdraw_bond is blocked when the bond is locked for a pending inspection dispute (BondLocked).
• execute_slash reduces the inspector's bond by exactly slash_amount.
• A slash for more than the inspector's bond balance fails with InsufficientBond; bond untouched.
• slashing_module.slash is only callable by the registered bond_collateral contract address.
• cargo test -p bond_collateral and cargo test -p slashing_module pass.

Test plan

• cargo test -p bond_collateral — 14 tests pass (2 existing + 12 new inspector-bond tests covering full lifecycle, lock/unlock, cross-contract slash, multi-slash accumulation, every auth error variant).
• cargo test -p slashing_module — 12 tests pass (8 existing + 4 new for set_bond_contract, unregistered-caller rejection, non-positive-amount rejection, slash history recording).
• cargo test --workspace — no regressions in any other contract.
• cargo fmt --all -- --check clean.
• cargo clippy --workspace --all-targets --all-features clean (matches CI flags).

Closes #925

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