Delete legacy MNA builders — unified pipeline only (Phase B)

Remove 1320 lines: legacy build_passive_mna_stage,
build_orphan_output_mna_stage, try_build_multi_nl_stage, and the
legacy-mna feature flag. The unified sp_decompose + build_rtype_stage
pipeline is now the sole code path. Zero regressions confirmed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ajmwagar

pedalkernel/Cargo.toml

@@ -29,9 +29,6 @@ fault-injection = []
 # Detects when devices leave their valid linear operating region.
 # Zero-cost when disabled — all checks are behind #[cfg(feature)].
 runtime-warnings = []
-# Use the legacy per-type MNA builders instead of the unified sp_decompose + build_rtype_stage.
-# Enable for A/B comparison against the new pipeline. Will be removed once validated.
-legacy-mna = []
 # Tests that reference .pedal files from the private pedalkernel-pro repo.
 # Disabled by default so open-source consumers can still run all other tests.
 pro-pedals = []