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_prompt_caching.py

@@ -1,25 +1,26 @@
 """Prompt caching: the prompt has to start with the same bytes every turn.
 
-Every endpoint that caches prompts caches a *prefix* — it reuses the request up
-to the first byte that differs from last time and no further. So the cost of a
-turn is decided by layout: one section that changes each turn, placed near the
-top, re-prices everything underneath it, and underneath it is the story
-history, which is most of the prompt.
+Every endpoint that caches prompts caches a prefix. It reuses the request
+up to the first byte that differs from last time, and no further. The cost
+of a turn is therefore decided by layout: one section that changes each
+turn, placed near the top, re-prices everything underneath it, and
+underneath it is the story history, which makes up most of the prompt.
 
-Three things have to hold, and each is easy to undo by accident:
+Three things must hold, and each is easy to undo by accident:
 
-* the static block is byte-identical across turns — adding a section that moves
-  (live stats, retrieved memories, a rewritten summary) to `system_sections` is
-  the mistake this file exists to catch;
-* the sections that move sit *after* the history, but still *before* the tail
-  that is last for its own reasons (front memory, the length hint, and
-  EMIT_REMINDER, which is what keeps the state block emitted at all);
-* moving a section out of the system block does not drop it from the token
-  budget — it is still in the prompt.
+* The static block is byte-identical across turns. Adding a section that
+  moves (live stats, retrieved memories, a rewritten summary) to
+  `system_sections` is the mistake this file exists to catch.
+* The sections that move sit after the history, but still before the tail
+  that is last for its own reasons: front memory, the length hint, and
+  `EMIT_REMINDER`, which is what keeps the state block emitted at all.
+* Moving a section out of the system block does not drop it from the
+  token budget. It is still in the prompt.
 
-Plus the two request-level halves: preferring one OpenRouter upstream, since
-each upstream holds its own cache, and reading back the usage the endpoint
-reports so the hit rate is measurable rather than assumed.
+This file also covers two request-level concerns: preferring one
+OpenRouter upstream, since each upstream holds its own cache, and reading
+back the usage the endpoint reports, so the hit rate is measurable rather
+than assumed.
 
     python -m pytest tests/test_prompt_caching.py -v
 """
@@ -68,8 +69,8 @@ def test_fallbacks_stay_on():
 
 
 def test_non_openrouter_endpoints_get_no_provider_field():
-    """Ollama and friends reject fields they do not know — the same trap the
-    `reasoning` param is written around."""
+    """Ollama and other providers reject fields they do not know. This is
+    the same problem the `reasoning` param works around."""
     body = _routed("http://localhost:11434/v1", "deepseek/deepseek-v4-flash-0731")
     assert "provider" not in body
 
@@ -85,8 +86,9 @@ def test_unknown_vendors_are_left_alone():
 # ------------------------------------------------------ reading usage back
 
 def test_usage_is_recorded_from_a_final_chunk():
-    """In a stream the usage block rides on a last chunk carrying no choices,
-    which is why it is read separately from the text extraction."""
+    """In a stream, the usage block arrives in a final chunk that carries
+    no choices, which is why it is read separately from the text
+    extraction."""
     provider = OpenAICompatibleProvider("https://openrouter.ai/api/v1", "k", "m")
     assert provider.last_usage is None
     provider._record_usage({"choices": [{"delta": {"content": "hi"}}]})
@@ -109,8 +111,8 @@ def test_a_later_chunk_without_usage_does_not_erase_it():
 # ------------------------------------------------------------ prompt layout
 
 def _with_hp(world_state, hp):
-    """`world_state` is nested by group, and the JSON column only notices a
-    whole new object — so build one rather than mutating in place."""
+    """`world_state` is nested by group, and the JSON column only detects a
+    whole new object. Build a new one instead of mutating in place."""
     return {**world_state, "player": {**world_state["player"], "hp": hp}}
 
 
@@ -153,8 +155,9 @@ def story():
 
 
 def test_changing_a_stat_leaves_the_static_block_untouched(story):
-    """The whole point. Live values used to sit third from the top, so a single
-    point of damage re-priced the instructions, the plot and the history."""
+    """The whole point. Live values used to sit third from the top, so a
+    single point of damage re-priced the instructions, the plot, and the
+    history."""
     db, adventure, settings = story
     before, _, _ = builder.build_context(adventure, settings)
     adventure.world_state = _with_hp(adventure.world_state, 40)
@@ -169,8 +172,8 @@ def test_the_static_block_holds_the_things_that_do_not_move(story):
     system_text, story_text, _ = builder.build_context(adventure, settings)
     for fixed in ("Write in second person.", "The hero hunts bandits."):
         assert fixed in system_text
-    # The stat *guide* is derived from the schema and so is fixed; the live
-    # values it describes are not, and belong to the story text.
+    # The stat guide is derived from the schema, so it is fixed. The live
+    # values it describes are not fixed, and belong to the story text.
     assert "Stat guide" in system_text
     for moves in ("The hero left the village.", "hp 100/100"):
         assert moves not in system_text
@@ -186,8 +189,9 @@ def test_volatile_sections_sit_after_the_history(story):
 
 
 def test_the_tail_stays_the_tail(story):
-    """front memory, the length hint and EMIT_REMINDER are last for reasons of
-    their own, and the live sections must not have displaced them."""
+    """Front memory, the length hint, and `EMIT_REMINDER` are last for
+    reasons of their own, and the live sections must not have displaced
+    them."""
     db, adventure, settings = story
     _, story_text, report = builder.build_context(adventure, settings)
     labels = [s["label"] for s in report["sections"]]
@@ -199,8 +203,8 @@ def test_the_tail_stays_the_tail(story):
 
 def test_live_sections_are_still_charged_to_the_budget(story):
     """They moved out of `system_sections`, so it would be easy to stop
-    counting them in `reserved` — and then the history, which is budgeted with
-    what is left over, would quietly overrun."""
+    counting them in `reserved`. If that happened, the history, which is
+    budgeted with what is left over, would quietly overrun."""
     db, adventure, settings = story
     for i in range(6, 90):
         db.add(models.Action(