tests: slashing-conservation property tests (staking)

[adamkrellenstein]

Stacked on #440. Adds seeded-random conservation property tests over the slashing surface — slashing is security-critical and adversarially exposed (the audit found a real self-publication leak here), so it warrants invariants beyond the existing point tests.

Tests (mirror the existing distribute_ordering_reward_conserves pattern)

• slash: burned + redistributable == slashed; burned is exactly the τ_slash 50% share; slashed == min(amount, stake) (saturates, never negative); stake drops by exactly slashed.
• distribute_slash: Σ recipient stake == n·base + amount (exact, no dust).
• slash_equivocation: λ_equiv=100% (full stake taken); burned + bounty == slashed; offender zeroed/ejected; self-publication ⇒ no bounty.

Design decisions

• Seeded StdRng-style loop, not proptest — matches the existing *_over_random_cases template and the lite-harness style (fresh test_runtime_with_genesis per case, Signer::Core calls, post-state asserts). No new dependency.
• Conservation checked via +/== only (e.g. burned + burned == slashed rather than a * or -), since the host Decimal operator surface used elsewhere in these tests is +/comparison.

Test-only; no contract change.

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Note

Low Risk
Test-only additions to indexer contract tests; no production staking logic is modified.

Overview
Adds seeded-random conservation property tests in staking_slash.rs for the staking slashing APIs, using the same lite-runtime *_over_random_cases style as distribute_ordering_reward_conserves_over_random_cases (fresh genesis per seed, Decimal checked with +/== only).

slash: many cases assert slashed == min(amount, stake), burned + redistributable == slashed, 50% burn via burned + burned == slashed, and stake reduced by exactly slashed (including amounts above stake).

distribute_slash: asserts total recipient stake equals n·base + amount with no dust.

slash_equivocation: full-stake slash, burned + bounty == slashed, offender stake zeroed, and alternating self vs external publisher (no bounty on self-publication, positive bounty otherwise).

Test-only; no contract or runtime behavior changes.

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