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/dbmeter.py

@@ -4,7 +4,8 @@ Query *counts* are easy to see and have never been the problem here. Both
 egress blowouts this project has had were one query fetching a column nobody
 read: `context_snapshot` on every action, then every memory's embedding on
 every turn. Counting statements would have shown nothing wrong in either case,
-and at development scale — ten rows, no embeddings — so would a stopwatch.
+and at development scale, meaning ten rows and no embeddings, so would a
+stopwatch.
 
 So this meter measures bytes, and it measures them at the only place the truth
 is available: the DBAPI cursor, after the driver has decoded a row and before
@@ -20,9 +21,9 @@ Usage:
     print(meter.render())
 
 `attach()` wraps the pool's connection factory, so it reuses the engine the app
-already configured rather than rebuilding one beside it — nothing about
-connect args, pre-ping or the SQLite foreign-key pragma has to be repeated
-here, and drift between the metered engine and the real one is impossible.
+already configured rather than building a second one. Nothing about connect
+args, pre-ping, or the SQLite foreign-key pragma is repeated here, and the
+metered engine cannot diverge from the real one.
 Existing pooled connections are dropped first, so a connection opened before
 attaching cannot quietly stay unmetered.
 
@@ -68,8 +69,8 @@ def value_bytes(value) -> int:
     if isinstance(value, (int, float)):
         return 8
     if isinstance(value, (dict, list)):
-        # A JSON column the driver already parsed (psycopg does; SQLite does
-        # not). Separators match what a database emits — no spaces.
+        # A JSON column the driver already parsed. psycopg parses it and SQLite
+        # does not. The separators match what a database emits, with no spaces.
         return len(json.dumps(value, separators=(",", ":"), default=str).encode("utf-8"))
     return len(str(value).encode("utf-8"))
 
@@ -82,8 +83,9 @@ def _table_of(statement: str) -> str:
     match = _TABLE_RE.search(statement)
     if match:
         return match.group(1).lower()
-    # No table to name — BEGIN, a PRAGMA, a savepoint. Group those under the
-    # keyword so they stay countable instead of collapsing into one "?" bucket.
+    # The statement names no table, such as BEGIN, a PRAGMA, or a savepoint.
+    # Group those under the keyword, so they stay countable rather than collapse
+    # into one "?" bucket.
     head = statement.strip().split(None, 1)
     return head[0].lower()[:20] if head else "(empty)"
 
@@ -236,10 +238,10 @@ class _MeteredConnection:
 class _MeteredCursor:
     """Counts the bytes of every row handed back.
 
-    The fetch methods are wrapped rather than `execute`, because what a
-    statement *costs* is not knowable when it is sent — `SELECT * FROM
-    memories` and `SELECT count(*) FROM memories` look alike going out and
-    differ by three megabytes coming back.
+    The wrapper covers the fetch methods rather than `execute`, because what a
+    statement costs is not known when it is sent. `SELECT * FROM memories` and
+    `SELECT count(*) FROM memories` look alike going out and differ by three
+    megabytes coming back.
     """
 
     def __init__(self, cursor, meter: Meter) -> None: