M1: IR Blend, Distance emulation, inter-IR phase alignment, broadened tests, docs

[yves-vogl]

Summary

Implements milestone M1 - DSP completion & test coverage for Nave and prepares the v0.1.0 release.

DSP (issue #1, partially - see below)

• IR Blend: a second, independently loadable impulse-response slot (IR B), crossfaded against the original slot (IR A) via a new IR Blend parameter. Defaults to 0% (IR A only) - bit-identical to the pre-M1 single-IR signal path, so no existing preset's sound changes.
• Inter-IR phase alignment: loading IR B automatically time-shifts it (src/dsp/IrAlignment.{h,cpp}) so its transient onset matches IR A's most recently loaded onset, before the two are ever blended - prevents comb-filtering from a timing mismatch between independently captured IRs. Alignment is computed in time, not raw samples, so it stays correct across differing IR sample rates.
• Distance: a simulated mic-to-cab distance control (proximity-effect low-shelf + air-absorption high-shelf, both scaling with the parameter), applied post-convolution/pre-LoCut. Defaults to 0% ("off"), using the same explicit-bypass-at-the-extreme pattern as LoCut/HiCut, so the default state is still a bit-exact passthrough.
• Updated signal flow: Convolution (IR A/IR B blend) -> Distance -> LoCut -> HiCut -> Mix -> Level.
• Both IR slots' file paths and the two new parameters round-trip through getStateInformation/setStateInformation.
• Editor updated with IR B load/default controls and IR Blend/Distance knobs.

Deferred: the "IR browser + bundled IR library" part of issue #1 is not implemented in this pass - see the issue comment for the full rationale (asset-sourcing/licensing concern, not a DSP task; better scoped into M2/M3). Issue #1 is left open.

Test coverage (issue #2 - closes)

• New tests/IrAlignmentTests.cpp: onset detection, sample shifting, cross-sample-rate alignment.
• New engine-level tests for IR Blend (0%/50%/100%) and Distance (passthrough at 0%, measurable attenuation at 100%).
• New tests/CoverageTests.cpp: sample-rate sweep (44.1-192 kHz) null + finite-output tests, mono/stereo/unsupported bus-layout tests, and long-run (2000-block, and 300-block-with-loaded-IRs) NaN/Inf stability soak tests.
• Existing parameter/state/latency tests extended for the two new parameters and the IR B file path.
• The original null/reference and latency tests are untouched in behaviour and stay green throughout.

50 Catch2 test cases green (up from 22), each well under a second locally.

Documentation

• New docs/manual.md: full user manual (what Nave is, signal flow, complete parameter reference, usage tips).
• README.md: updated feature list, signal-flow diagram, parameter table, and roadmap (now matching the actual GitHub milestones M1-M4).
• docs/architecture.md: updated signal-flow diagram, new sections on IR Blend/phase alignment and Distance emulation, updated latency/smoothing/real-time-safety sections for the two-slot convolution engine.
• CLAUDE.md: Status/DSP sections updated.

Versioning

• CMake project version was already 0.1.0 (no change needed).
• CHANGELOG.md: moved Unreleased content into a [0.1.0] - 2026-07-14 section documenting everything above, including the deferral.

Test evidence

export CPM_SOURCE_CACHE="$HOME/.cache/CPM"
cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug
cmake --build build --target Tests Nave_Standalone --parallel 3
ctest --test-dir build --output-on-failure

• Tests and Nave_Standalone build cleanly (no warnings).
• ctest: 100% tests passed, 50/50, ~11s total wall time.

Closes

Closes #2