tests: storage-economics formula + conservation property tests

[adamkrellenstein]

Stacked on #441. Adds property invariants for the storage emission weights (ω_f, k_f), the Ω accumulator, and reward distribution — mirroring the modeling test_storage.py oracle.

Tests

• storage_economics_invariants_over_random_sizes — over 12 random file sizes: ω_f/k_f strictly positive; rank_f increments by exactly 1 per creation; inactive creates leave Ω unchanged; each activation strictly increases Ω and increments |F|; distribute_storage_rewards conserves (Σ allocations ≤ pool, each positive).
• storage_omega_f_decreases_with_rank_at_equal_size — equal size + higher rank ⇒ strictly smaller ω_f (ω_f = log10(s)/log10(1+rank)).

Design decisions

• Fresh-runtime #[testlib::test] (test_file_storage_storage_properties) so the property fns aren't polluted by storage_economics_smoke's state; the fns are also self-relative (capture starting Ω/|F|).
• Structural invariants via comparisons, not re-derived exact ω_f/k_f — robust to fixed-point rounding; the smoke test + Python oracle pin exact values.

Test-only; no contract change.

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Note

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

Overview
Adds local-only indexer contract coverage for filestorage storage economics via a dedicated test_file_storage_storage_properties entry point, separate from the existing smoke test so runtime state stays clean.

New property checks exercise the native contract through create/join/activate flows: over many random file sizes they assert ω_f and k_f stay positive, rank_f steps by one per agreement, inactive creates leave Ω unchanged, each activation strictly raises Ω and |F|, and distribute_storage_rewards returns only positive allocations with total paid ≤ the pool. A second case pins that equal s_bytes with higher rank_f yields strictly smaller ω_f (rank penalty in the weight formula).

Assertions are structural (monotonicity, conservation, ordering) rather than re-deriving exact fixed-point weights, aligning with the Python modeling oracle without duplicating rounding logic in Rust.

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Merge order: #456 and #452 both modify core/indexer/tests/contracts/file_storage_tests/native_filestorage_contract.rs. Merge impl PR #452 before this test PR; rebase this on top afterward.