Docs: module notes from open reports (FM Synth, USB Audio, Euclid, Vector) - #701

• What was wrong or hard: Five open reports describe behaviour the module pages don't explain, so people hit it with no warning and no workaround.
  • FM Synth clicks with ATK 0 (related to [synth] Click #557).
  • FM Synth's LFO1/LFO2 show FLEX's destination names (related to [synth] Wrong LFO1&2 destination names #589).
  • USB Audio leads to OUT OF MEMORY in projects that filled Flex memory (related to [usb-audio-out-tracks-main-cue] Flex machine slot “out of memory” error #433).
  • Euclid at MIX 127 can sound dry (related to [euclid] Mix knob not 100% wet? #681).
  • Vector can leave a bank empty after you switch a track back to STATIC/FLEX and reboot (related to [configuration] When loading the saved project, the sequencer is emptied. #596).
• What this changes: Documentation only, with no firmware or version changes. Each module's README and the matching compatibility.limitations or control doc text in its manifest get the note and the workaround. Each module's current changelog entry gets a marked "Documentation-only (19 October 2026)" note. module-documents.json is regenerated.
  • FM Synth: ATK 0 with a short HOLD or REL can click; raise ATK or lengthen REL. LFO1/LFO2 list FLEX names, each standing for the FM knob in the same position (STRT is RATO, LEN is INDX, RATE is FINE, RTRG is FDBK, RTIM is DEC). Whether LFOs on them change the FM sound is not tested.
  • USB Audio: like every DRAM module, the build gives up about 10 MB of sample memory. Shorten the recorder reservation or RLEN, or load long samples into STATIC. [usb-audio-out-tracks-main-cue] Flex machine slot “out of memory” error #433 stays open in case there is also a separate allocation problem.
  • Euclid: MIX 127 is fully wet, but the wet signal is the input through the pulsed filter or amp. With the filter open or DEPTH at center it can sound dry.
  • Vector: the 0.2.4 fix covers only tracks still on VECTOR. In native.c, st_assign clears only the three marker bytes on deselect. At SPAN 8 or higher, the byte vector_pack leaves at sig+5 is the one the stock check rejects, so the bank can load empty after a save and reboot. This is read from the source and not reproduced. Workaround: set SPAN to 7 or lower before switching back, and don't SAVE a bank that already loads empty. The real fix, clearing all twelve bytes and tolerating the leftover byte on load, needs a firmware version and is not part of this PR.
• How I checked it: npm run check -- --base origin/main and npm run pr:ready (Ready; 2,353 tests passed). Before writing the notes I read the FM page descriptor (upstream/synth/page.s) and Vector's engine.c and native.c. Nothing was run on a unit or in the emulator.

FM Synth's open update #555 also touches sdk/octabam/modules/synth/. The edits here are additive lines in the README and manifest, so any conflict should be small.

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