Add per-operator phase offset (note-on start phase) with UI + preset compatibility

add a dedicated phase offset parameter for each operator
apply operator phase offset on note-on in voice init path
add operator-page UI state to edit phase offset with short labels
keep editor architecture intact by using dedicated operator phase rows
extend runtime and flash parameter structs for phase persistence
update save/load conversion for new phase parameters
preserve compatibility for existing/legacy patches (including v2 layout handling)
update built-in preset initializers with default phase entries
add/refresh code comments in changed areas for maintainability

DX7 import: validate sysex bank and align importer mapping

harden DX7 sysex loading by reading full 32-voice bank frames and validating:
- frame size and header/footer bytes
- MIDI channel nibble format
- 7-bit payload/checksum bytes
Yamaha checksum integrity
reject invalid patch indices and invalid bank data before import.
add null-safe DX7 patch loading path in synth state to avoid loading invalid pointers.
rework DX7 feedback import:
- map DX7 feedback to modulationIndex6 (capped at 0.9)
- remove legacy fb>4 saw-shape workaround branches
fix transpose import branch ordering so deep negative transpose is reachable.
fix AM depth regression:
- preserve row 2 AMD-derived modulation amount
- add regression guard comment
improve IM velocity split mapping with updated KVS ratios and apply split only when KVS > 0.
import DX7 LFO waveform and key sync in importer:
- map DX7 wave shapes to internal LFO shapes
- map key sync state into keybRamp sign semantics
- deduplicate shared LFO import assignment logic
clamp DX7 PMD/AMD to 0..99 before scaling/table lookup.
add compile-time table-size guard for DX7 AMD lookup table.
fix LFO switch-case scope issue in header after key-sync update.
refresh and correct README/docs wording and links.