Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/tools/measure_bundle.py

@@ -16,12 +16,12 @@ import json
 import random
 import sys
 
-# `stress_session` FIRST, and it is not a style preference. Importing it is what
-# points `AIDND_DB_PATH` at a throwaway file, and `app.database` reads that at
-# module scope — so an `app` import above this line silently runs the whole
-# fixture against `backend/data.db` instead. It fails by *working*: the first
+# Import `stress_session` first. This is not a style preference. Importing it is
+# what points `AIDND_DB_PATH` at a throwaway file, and `app.database` reads that
+# at module scope, so an `app` import above this line runs the whole fixture
+# against `backend/data.db` instead. The failure looks like success: the first
 # run seeds a synthetic user and adventure into the local database and reports
-# perfectly good numbers, and only the second run trips over the unique email.
+# plausible numbers, and only the second run fails on the unique email.
 from tools import stress_session as stress  # noqa: I001  (see above)
 
 from app import bundle, models, tree
@@ -31,8 +31,9 @@ from app.database import SessionLocal
 def _v1_shape(v2: dict) -> dict:
     """The same story as v1 would have written it, for a like-for-like count.
 
-    One entry per turn, the siblings folded back into a `variants` array, no
-    coordinates and no outcomes — which is exactly what v1 could carry.
+    There is one entry per turn, with the siblings collected back into a
+    `variants` array, no coordinates, and no outcomes. That is what version 1
+    could carry.
     """
     turns: dict[tuple[int, int], list[dict]] = {}
     order: list[tuple[int, int]] = []
@@ -89,11 +90,12 @@ def main(argv=None) -> int:
     try:
         adventure = db.get(models.Adventure, adv_id)
         if forks:
-            # Twenty divergences off one line, each a little deeper — the shape
-            # SP5 measured the fork cost on. The nodes are chosen up front and
-            # the session is flushed after every fork: `fork` moves a row onto
-            # its new branch, and with `autoflush=False` a query issued before
-            # that move is written still finds the node where it used to be.
+            # Twenty divergences off one line, each a little deeper, which is
+            # the shape SP5 measured the fork cost on. The nodes are chosen up
+            # front and the session is flushed after every fork. `fork` moves a
+            # row onto its new branch, and with `autoflush=False` a query issued
+            # before that move is written still finds the node at its old
+            # location.
             root = adventure.head_branch_id
             candidates = [
                 node.id for node in