docs(presets): document every shipped factory preset

[yves-vogl]

Summary

• docs/presets.md described 9 of the 12 factory presets shipped in presets/factory/*.json. Added rows for the three v0.3.0 additions that were missing: Circuit Foundation, Circuit Grind, and Circuit Knife.
• Each new Intent description is derived from the preset's actual stored parameter values, read against the plugin's defaults in src/params/ParameterLayout.cpp and cross-checked against docs/manual.md's Circuit-engine parameter reference - not guessed from the preset name.
• Removed the now-resolved "not described below yet" caveat from the intro.

What each preset actually does (from stored values, not the name)

• Circuit Foundation: all three new v0.3.0 engine selectors (Drive Engine, Low Comp Detector, Gate Mode) left at their new defaults, with a widened mid band, soft-kneed parallel-blended low glue and restrained mid/high drive - the baseline Circuit-engine tone.
• Circuit Grind: firm, fast low-band compression under aggressive mid/high drive with heavy even-harmonic High Bias - the aggressive end.
• Circuit Knife: a light touch on the low band but the most extreme high band of the three (Gnaw voicing, Tight pushed to 200 Hz, Drive to 90%) for a narrow, cutting rather than warm top end.

Out of scope

The issue's Definition of Done also asks for a test asserting docs/presets.md and PresetManager::getAllPresets() can't drift apart again. That requires touching tests/, which is outside this PR's scope (docs-only). Flagging so a follow-up can add it.

Closes #117

Test plan

• Diffed every presets/factory/*.json name field against the new table - all 12 are present verbatim.
• Verified the three new Intent rows against src/params/ParameterLayout.cpp defaults and docs/manual.md's Circuit-engine parameter reference.
• CI (docs-only change, no build/test impact expected)