[BUG] Partial mixer inserts from some FL projects can crash snapshot extraction

[aernw]

Objective

Track parser-side compatibility work for FL Studio projects that produce partial mixer inserts and currently rely on StemHub-side defensive fallbacks.

Problem

SuperBeat.zip reproduced a parser compatibility gap during FL snapshot extraction.

load_fl_studio_mixer_snapshot() successfully extracts and parses SuperBeat.flp, but some parsed mixer inserts are missing params in the PyFLP insert model. That causes Insert.__iter__() to raise KeyError: 'params' during slot extraction.

Evidence gathered locally:

• archive: backend/tests/fixtures/parser_corpus/assets/stemhub_snapshots/SuperBeat.zip
• extracted FLP: SuperBeat.flp
• failing inserts: mixer insert ids 61 and 62
• parser warning observed during parse: VSTPluginEvent: Unknown marker 12 detected; continuing parse with best-effort fallbacks.
• parser stack failure before StemHub hardening:
  • backend/vendor/PyFLP_v2/pyflp/mixer.py:413
  • yield Slot(ed, params=self._kw["params"].slots[idx])
  • KeyError: 'params'

StemHub was hardened to tolerate this shape and continue building a snapshot, but the underlying issue is still parser/model incompleteness for this FLP.

Why This Matters In M2

M2 is FL-core and storage/diff heavy. We need parser behavior that is resilient enough for real user projects, especially when snapshot extraction feeds compare/history flows. A parser that yields partial mixer inserts without a stable compatibility contract will keep surfacing as flaky diff coverage or silent semantic loss.

Scope

• investigate why this FLP yields inserts without params
• determine whether the unknown VST marker path is the direct cause
• define parser behavior for partial mixer inserts:
  • either populate safe/default insert params
  • or expose a documented partial/unsupported state without crashing downstream iteration
• add an upstream-compatible regression fixture or reproduction path for this project shape

Out Of Scope

• redesigning StemHub diff semantics beyond necessary parser compatibility handling
• broad plugin compatibility work unrelated to this failure pattern

Acceptance Criteria

• SuperBeat.flp no longer produces crashing mixer inserts during slot iteration
• parser behavior for inserts missing parameter payloads is explicit and test-covered
• StemHub no longer needs to rely on this exact defensive fallback to keep snapshot extraction alive for this project shape

Current StemHub Status

• StemHub corpus now includes SuperBeat.zip as a regression fixture
• StemHub snapshot extraction no longer crashes on this archive
• StemHub still receives the parser warning and best-effort partial mixer data

Suggested Labels / Priority

Parser compatibility bug affecting FL snapshot extraction and downstream diff reliability.