Complete and refine the DSP

[yves-vogl]

Add an IR browser + bundled IR library, dual-IR blending, mic-position/distance emulation, and inter-IR phase alignment. Keep the delta-IR passthrough null test.

[yves-vogl]

Status update from the M1 DSP pass (PR incoming): implemented IR Blend (a second, independently loadable IR slot crossfaded against the original one), inter-IR phase alignment (loading IR B automatically time-shifts it to match IR A's transient onset, so blending never introduces comb-filtering from a timing mismatch), and Distance (a simulated mic-to-cab distance control - proximity-effect low-shelf + air-absorption high-shelf, off by default).

Deferring the "IR browser + bundled IR library" part of this issue and leaving it open. Reasoning: a genuinely useful bundled library needs either licensed real-world cabinet captures (an asset-sourcing/licensing task, not DSP engineering) or synthetic placeholder IRs that risk being mistaken for real captures if shipped without very careful labelling - neither is something to rush through in a DSP-focused pass. It's also naturally closer to M2 (presets/factory content) and M3 (GUI - an actual browsing UI) than to M1's DSP scope. Recommend re-scoping this remaining piece into M2/M3, or tracking it as its own follow-up once IR assets/licensing are sorted out.

The delta-IR passthrough null test stays green throughout (see tests/EngineTests.cpp, tests/CoverageTests.cpp).