tests: congestion β(t) region/bound invariant property tests

[adamkrellenstein]

Stacked on #445. Adds property tests for the β(t) congestion recurrence.

Tests

• congestion_beta_invariants_over_random_walk — 60-step random walk of utilizations across all three regions, asserting: region 1 decays (β'≤β); region 3 grows (β'≥max(β,1)≥1); region 2 bounded by the smoothstep ceiling (β'≤max(β,1)); β never negative; fee_multiplier == φ_base·β throughout.
• congestion_smoothstep_continuous_at_band_edges — S(1)=1 at u_high (joins region 3's floor), S(0)=0 at u_low (joins region 1's decay floor).

Design decision

The random-walk test asserts structural invariants via comparisons (≤/≥) rather than re-deriving exact β' values. Region-2's x = (u−u_low)/(u_high−u_low) is not always a terminating decimal, so exact re-derivation would couple the test to fixed-point rounding; the existing point tests already pin exact interior values (1.2 → 1.44 → 1.368, midpoint 0.5). The Python test_congestion.py remains the value oracle.

Test-only; no contract change.

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Note

Low Risk
Test-only additions to contract tests; no production, auth, or fee logic changes.

Overview
Adds two in-process contract tests for the congestion β(t) recurrence in congestion.rs, complementing existing point-value checks.

congestion_beta_invariants_over_random_walk runs 60 deterministic utilization steps (0–100% in bps) and asserts structural properties via inequalities: low-util decay (β′ ≤ β), high-util growth (β′ ≥ 1 and ≥ prior β), mid-band smoothstep ceiling (β′ ≤ max(β, 1)), non-negative β, and fee_multiplier == φ_base · β each step—without pinning exact β′ in region 2 where fixed-point rounding would be brittle.

congestion_smoothstep_continuous_at_band_edges checks continuity at u_low / u_high: S(1)=1 at the upper band (β→1 from zero) and S(0)=0 at the lower band with the region-1 decay floor (β→0.95 from β=1).

No runtime or contract logic changes; Python remains the value oracle for exact numbers.

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