refactor: cleanup, dedup, and optimize Rust solver

[daharoni]

Summary

• Correctness fix: Extract clamp_tau_rise() guard and apply in BandedAR2, fixing a case where banded and FFT engines disagreed for tau_rise ≈ tau_decay
• 6 deduplication refactors: fft.rs forward/adjoint, banded.rs new/update, baseline EMA, kernel_est adjoint, biexp_fit golden-section, indeca interior_slice
• 8 performance improvements: fused banded normalize, halved Lipschitz loop, lazy banded update, hoisted loop allocations, cached FenwickTree MSB, combined median+MAD, moved ownership instead of clone
• 3 new filter-path tests covering HP+LP, HP-only, and LP-only paths in indeca::solve_trace
• WASM binary shrunk 9KB (416→407 KB)

Files changed (14)

File	Changes
kernel.rs	Extract clamp_tau_rise(); halve compute_lipschitz loop
banded.rs	Add tau guard; collapse update(); fuse normalize pass
fft.rs	Extract convolve_impl() shared by forward/adjoint
lib.rs	Extract baseline EMA + state_byte_len; lazy banded update
fista.rs	Use extracted baseline EMA method
filter.rs	Extract forward_fft_and_cache_power(); iterator-zip
baseline.rs	Cache MSB in FenwickTree
kernel_est.rs	Hoist allocs; extract adjoint_spikes_kernel()
biexp_fit.rs	Extract golden_bracket(); sentinel(); has_fast_component(); fix comment
threshold.rs	sort_unstable_by+total_cmp; move s_bin; fuse PVE
peak_seed.rs	Combined median_and_mad()
indeca.rs	Direct filter; interior_slice(); 3 new tests
js_indeca.rs	Formatting only (rustfmt)
pkg/calab_solver_bg.wasm	Rebuilt (-9KB)

Test plan

• All 109 Rust tests pass (106 existing + 3 new)
• WASM builds clean
• cargo fmt --check passes
• Verified all existing test assertions still align with refactored code

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