Refactor/Add convergence-rate-based adaptive frequency for BPCG subspace diagonalization

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Summary

This PR adds a convergence-rate-based adaptive frequency mechanism to the BPCG diagonalization method. The subspace diagonalization call frequency now dynamically adjusts based on the actual convergence state, building on top of the existing problem-size-based frequency.

Key Idea

Track historical error changes, compute convergence rate every 2 CG steps, and adjust frequency accordingly:

• Fast convergence → reduce diagonalization (save computation)
• Slow/stagnant convergence → increase diagonalization (accelerate convergence)

Convergence Rate Definition

conv_rate	State	Action
< 0.3	Very fast	freq + 1
0.3 - 0.6	Fast	keep baseline
0.6 - 0.85	Moderate	freq - 1
≥ 0.85	Stagnant	freq - 2

Two-Level Frequency Control

Level	Based On	When
Level 1	Problem size (n_band × n_basis)	Once at diag() entry
Level 2	Convergence rate (error trend)	Every 2 CG steps

Final frequency = baseline_freq ± adaptive_adjustment (clamped to [1, nline])

Code Changes

File	Change
bpcg_kernel_op.h/cpp	Add compute_convergence_rate() and compute_adaptive_freq()
diago_bpcg.h	Add 4 member variables + update_adaptive_freq()
diago_bpcg.cpp	Integrate adaptive frequency into CG loop

Benefits

• Fast convergence: Reduced unnecessary diagonalization overhead
• Slow convergence: More frequent resets to maintain orthogonality
• Backward compatible: No interface changes, default behavior unchanged

Example Usage

The adaptive frequency automatically responds to convergence behavior without any user configuration.