[REFACTOR] Fix root-cause type annotations causing mypy error fan-out

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🎯 Objective

A small number of type-annotation root causes are responsible for a disproportionate share of the 235 mypy errors measured in #429. Fix these first, before the file-by-file cleanup in the other sub-issues -- the fan-out should shrink the total error count significantly with a handful of targeted, low-risk changes.

🛠️ Known root causes (confirmed, not guessed)

• `RECORDING_SCHEMAS` values typed as `Collection[str]` instead of `List[str]` (`gitgalaxy/standards/analysis_lens.py`) -- `SIGNAL_SCHEMA`/`RISK_SCHEMA` are built via `config.RECORDING_SCHEMAS.get(...)`, and `Collection` doesn't guarantee `.index()`. This alone produces ~15+ `"Collection[str]" has no attribute "index"` errors across `galaxyscope.py`, `record_keeper.py`, `signal_processor.py`, and others.
• `import importlib` without `import importlib.util` (`gitgalaxy/galaxyscope.py:70`) -- `importlib.util.find_spec` is used but only `importlib` is imported; works at runtime today only because something else transitively imports `importlib.util` first, which is fragile. One-line fix.
• Heterogeneous dict literals without a TypedDict (e.g. `record_keeper.py`'s `folder_stats[p]` -- a dict mixing `int`, `float`, and `list` values) -- mypy collapses per-key access to `object`, producing `"object" has no attribute "append"` / `Unsupported operand types for +` errors. Needs a small local `TypedDict` per such structure, not a real bug fix.

Note

Not everything under the `assignment`/`arg-type`/`return-value` codes is mechanical -- at least one (`record_keeper.py:512`, `None` assigned to a `float`-typed variable) looks like it could be a genuine latent bug. Review each individually rather than blanket-suppressing.

Parent: #429

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All of this issue's explicitly-named root causes are fixed:

• `RECORDING_SCHEMAS` (`analysis_lens.py`) -- added a `RecordingSchemas` `TypedDict`. Resolved 42 fan-out errors across `galaxyscope.py`, `signal_processor.py`, `record_keeper.py`, `prism.py` without touching those files.
• `import importlib` without `import importlib.util` (`galaxyscope.py:70`) -- fixed.
• Heterogeneous dict literals needing a `TypedDict` -- `record_keeper.py`'s `folder_stats[p]` (the example this issue named directly) got a `FolderStats` `TypedDict`, resolving 15 errors. The same pattern also showed up in `language_lens.py`'s `LENS_CONFIG` (not yet discovered when this issue was filed) -- same fix, `LensConfig` `TypedDict`, resolved 20 more.
• The `record_keeper.py:512` `None`-assigned-to-`float` case flagged as "review individually, don't blanket-suppress" -- confirmed genuine and intentional (zero-dependency mode maps to real SQL `NULL`, not `0.0`, to avoid an unavailable metric looking like a measured "zero risk"). Annotated `Optional[float]` rather than changing the `None`.

Net effect on the baseline: 228 → 126 unique entries (45%) since this epic started, though not all of that reduction is root-cause fan-out specifically -- some came from fixing files directly (see #432's progress).

Closing this one -- the specifically-scoped root-cause hunt is done. Any new root causes turned up while working through #432/#433 are the same class of fix, so I'll just apply them inline rather than reopening this.