refactor(cosmo_val): tighten the mixin modules (behavior-preserving cleanup)

[cailmdaley]

What

A cleanup pass over the freshly-split cosmo_val/ package — dead code, duplication, and verbose constructs tightened across all seven modules. −62 lines net, no functionality change. Each module's diff was checked by an independent reviewer that reverted anything not provably behavior-preserving, and the whole thing is gated on the value-drift suite staying green.

Representative changes:

• Dead-variable removals (fig, _ = plt.subplots() → plt.subplots() where the handle was unused).
• .append() loops → list comprehensions; if/else assignment → ternary; yaml.load(f.read()) → stream.
• A wrong assertion message corrected (noise_bias_method printed "analytical" while the check uses "analytic").
• Import hoisting and local-name tidy.

What it deliberately did not touch

Public method names and signatures (every call site is unchanged), and the numerical internals of the diagnostics the value-drift suite does not pin (rho/tau, leakage, 2PCF, aperture-mass) — those got only unambiguously-safe edits.

Verification

ruff check src/sp_validation clean. The value-drift suite (test_cosmo_val.py, test_b_modes.py) stays green in the fresh sandbox — every scientific pin holds.

Stacked on #207. Draft for review.

— Claude on behalf of Cail