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/tests/test_memory_settling.py

@@ -1,22 +1,24 @@
 """Memories must never describe narration that is no longer in the story, and
 must never skip a stretch of it.
 
-For six phases the answer was a **holdback**: summarization stopped one action
-short of the newest, because only the last action was retryable and a retry
-rewrote `Action.text` under a mark that had already moved past it. SP4 ended
-that — a retry writes a sibling node and the coordinate's derived work is
-withdrawn as it does, which is the same repair undo and delete already made.
-So the holdback is gone, and the first half of this file now asserts the
-property that replaced it: a block forms as soon as there is a block, and
-changing what a coordinate says takes back what was derived from it.
+For six phases, the answer was a holdback. Summarization stopped one action
+short of the newest, because only the last action was retryable, and a
+retry rewrote `Action.text` under a mark that had already moved past it.
+SP4 ended that: a retry writes a sibling node, and the coordinate's derived
+work is withdrawn as it happens, using the same repair that undo and delete
+already made. The holdback is gone, so the first half of this file now
+asserts the property that replaced it. A block forms as soon as there is a
+block, and changing what a coordinate says takes back what was derived from
+it.
 
-Phase 14 SP3 changed what the mark *is*. It used to be a count of covered story
-actions, and the second half of this file is the price of that: deleting an
-action from in front of a position slid a never-summarized action into the
-covered range, so every delete had to slide the cursors too. The mark is a node
-now — `(branch_id, depth)` — and a node does not move when something in front
-of it is deleted, so those tests assert that nothing happens where they used to
-assert that the right correction happened.
+Phase 14 SP3 changed what the mark is. It used to be a count of covered
+story actions, and the second half of this file is the cost of that.
+Deleting an action from in front of a position slid a never-summarized
+action into the covered range, so every delete had to slide the cursors
+too. The mark is a node now, `(branch_id, depth)`, and a node does not move
+when something in front of it is deleted. Those tests now assert that
+nothing happens, where they used to assert that the right correction
+happened.
 
     python -m pytest tests/test_memory_settling.py -v
 """
@@ -110,13 +112,14 @@ def run_memories(db, adventure, stub, monkeypatch):
 # --------------------------------------------------- no holdback, since SP4
 
 def test_a_block_forms_as_soon_as_the_story_holds_one(db, monkeypatch):
-    """Covered to action 5 with 12 actions: block 6-11 ends on the *newest*
+    """Covered to action 5 with 12 actions: block 6-11 ends on the newest
     action, and is summarized now rather than a turn later.
 
-    This is exactly the case the holdback existed to refuse. What makes it safe
-    is no longer that the block stops short — it is that a retry of node 11
-    would withdraw this memory on its way past (see
-    `test_deleting_a_summarized_node_withdraws_its_memory`, the same repair).
+    This is exactly the case the holdback existed to refuse. What makes it
+    safe is no longer that the block stops short. It is that a retry of
+    node 11 would withdraw this memory on its way past (see
+    `test_deleting_a_summarized_node_withdraws_its_memory`, the same
+    repair).
     """
     adventure = make_adventure(db, 12)
     cover(db, adventure, 6)
@@ -128,8 +131,8 @@ def test_a_block_forms_as_soon_as_the_story_holds_one(db, monkeypatch):
     assert "Action 11." in stub.excerpts[0]
     memory = db.query(models.Memory).one()
     assert (memory.source_start, memory.source_end) == (6, 11)
-    # The mark and the memory name the same node — that is what keeps them from
-    # drifting apart however gappy the depths underneath are.
+    # The mark and the memory name the same node. That is what keeps them
+    # from drifting apart, however gappy the underlying depths are.
     assert (memory.branch_id, memory.depth) == cursors.MEMORY.stored(adventure)
     assert covered_depth(db, adventure) == 11
 
@@ -155,13 +158,13 @@ def test_the_first_memory_lands_at_memory_start(db, monkeypatch):
 
 
 def test_legacy_caught_up_adventure_is_not_rewound(db, monkeypatch):
-    """An adventure summarized under the OLD rule carries a cursor equal to its
-    action count — one past the end of the story. That used to need a clamp on
-    every post-turn pass, and clamping it to the settled count re-covered an
-    action.
+    """An adventure summarized under the old rule carries a cursor equal to
+    its action count, one past the end of the story. That used to require a
+    clamp on every post-turn pass, and clamping it to the settled count
+    re-covered an action.
 
-    A mark that names a node has no such edge: the newest action is the node,
-    and "everything after it" is empty until the story grows.
+    A mark that names a node has no such edge. The newest action is the
+    node, and "everything after it" is empty until the story grows.
     """
     adventure = make_adventure(db, 12)
     db.add(models.Memory(adventure_id=adventure.id, text="A", source_start=0, source_end=5))
@@ -238,9 +241,9 @@ def test_deleting_a_middle_action_leaves_the_mark_where_it_was(db):
     with node 5 gone.
     """
     adventure = summarized_adventure(db)
-    # Node 4 is inside memory A's block but is not the node it hangs off, so
-    # nothing is withdrawn — the same reading the old code had, where only a
-    # memory whose *end* had fallen off the story was pruned.
+    # Node 4 is inside memory A's block but is not the node it hangs off,
+    # so nothing is withdrawn. The old code read it the same way: only a
+    # memory whose end had fallen off the story was pruned.
     victim = db.query(models.Action).filter_by(adventure_id=adventure.id, index=4).one()
 
     assert memorybank.forget_node(db, adventure, victim) == 0
@@ -267,12 +270,13 @@ def test_deleting_a_later_action_leaves_the_mark_alone(db):
 
 
 def test_deleting_a_summarized_node_withdraws_its_memory(db):
-    """Discarding the memory isn't enough — the story it covered is still
-    behind the mark, so the mark has to come back to where that block began.
+    """Discarding the memory is not enough. The story it covered is still
+    behind the mark, so the mark has to move back to where that block began.
 
-    Memory B ends on node 11, so deleting node 11 is what withdraws it. The old
-    code found this by scanning for a memory whose covered range had fallen off
-    the end of the story; the memory hangs off the node now, so it is a lookup.
+    Memory B ends on node 11, so deleting node 11 withdraws it. The old code
+    found this by scanning for a memory whose covered range had fallen off
+    the end of the story. Now the memory hangs off the node, so finding it
+    is a lookup.
     """
     adventure = summarized_adventure(db)
     victim = db.query(models.Action).filter_by(adventure_id=adventure.id, index=11).one()