feat: v0.2.0 deep-dive voicing pass, M2 preset system, DE localisation

[yves-vogl]

Summary

Two things land together for v0.2.0, per the milestone plan (M2 = presets/state):

Research-derived deep-dive voicing pass (docs/design-brief.md, sourced/reasoned against docs/research-notes.md), retuning all four DSP stages without touching the topology or the zero-latency/bit-exact-bypass invariants:

• De-Ess: new DeEssWidth parameter (0-100%, default 40%) exposes the detector's bandpass Q as a user-facing control for the first time (0% → Q 3.0 narrow, 100% → Q 0.7 wide), replacing v1's hidden fixed Q = 1.2.
• Air: range narrowed and re-centered from ±12 dB to -6/+9 dB (default +2 dB), and the shelf's explicit Q lowered (~0.707 → 0.5) for a wider, gentler transition band.
• Gentle Compressor: fixed 150 ms release replaced by a program-dependent ("auto") release - a smoothed blend between a fast (~150 ms) and slow (~1.0 s) envelope path, transparent on isolated transients, glued on sustained program material.
• Doubler: base delays re-centered from 13/17/23/29 ms to 9/13/19/24 ms; DoubleDetune default lowered 15 → 10 cents with a reshaped power-law knob taper (cents = 50 * p^2.2, range unchanged) giving the tight-double register more knob travel. Not a breaking change to saved state (stored value stays cents; verified against JUCE 8.0.14's AudioProcessorValueTreeState source).

Every default/range change is either sourced with a citation in docs/research-notes.md or explicitly flagged "reasoned, not sourced" in docs/design-brief.md ss5. No brand/product names appear anywhere outside those two research documents (verified via repo-wide grep).

M2 preset system (.scaffold/specs/preset-system-m2.md), ported from the suite's pilot implementation in sibling plugin nave:

• src/presets/ - PresetManager (factory/user preset discovery, load/save/rename/delete, dirty tracking, default resolution, single-file and zip-bank import/export) and PresetBar (editor strip), copied verbatim per nave's replication recipe.
• Nine factory presets (presets/factory/*.json, documented in docs/presets.md): Default plus the brief's eight (Lead - Cut Through, Lead - Intimate/Close-Mic, Choir - Wide Spread, Choir - Tight Blend, Spoken/Growled Interlude, Glue Only, De-Ess Only (Surgical), Wide Double (No Dynamics)).
• German frame-string localisation (resources/i18n/de.txt), auto-selected from system language. Parameter names/units/DSP terminology are never translated.
• ICON_BIG was already wired by the preceding patch-release PR; verified still present.

Test plan

• cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug configures clean (JUCE 8.0.14 + Catch2 v3.15.2 via CPM cache).
• cmake --build build --target Tests Seraph_Standalone --parallel 4 builds clean.
• ctest --test-dir build --output-on-failure - 60/60 tests green (28 pre-existing + 32 new: 17 PresetManagerTests.cpp, 11 DeepDiveTests.cpp DSP-guarantee coverage, 1 StateTests.cpp tolerant-migration test, plus DeEssWidth/new-range coverage folded into existing Coverage/Robustness/Latency/Allocation tests).
• New DSP guarantees per docs/design-brief.md ss4: DeEssWidth bandwidth-curve monotonicity + Q-extremes (±10%, verified against JUCE's own analytic getMagnitudeForFrequency() to avoid digital pre-warping bias), DeEssWidth null-bypass sweep, Air magnitude-response shape (1/6/12/20 kHz) + null test, Comp auto-release ballistics (transient-vs-sustained recovery) + blend-boundary continuity + null test, Doubler delay-time bounds + null test + sample-rate sweep.
• Tolerant state migration: hand-built v0.1.0-shaped XML (no DeEssWidth) loads without failing or perturbing other parameters, DeEssWidth lands on its documented default.
• CI (macOS + Windows, pluginval strictness 10 + auval) - pending this PR's checks.

Known org-level issue (unrelated to this PR)

macOS release signing has a broken secret (tracked separately) - not exercised by this PR's CI (build/test only, no release job).