Serve a cache hit with no database read or write (closes #309)

A hit opens the variant file and nothing else: its content type comes
from the file's first bytes, and a file in no output format is a miss.
Its access time and hit count, and the access time of a cached source a
miss uses, go without waiting to the goroutine that wrote late counts,
which now writes them all once a second; each kind of access time waits
in a channel of 10,000 that drops the rest. checkNegativeCache leaves
expired rows to CleanExpired. The .meta files, metaCache, Lookup and
golang-lru go, with their tests. Dropping the .meta skip made the
variant and source reconciliation walks identical, so they share one.
make bench runs a parallel cache hit benchmark on the host, outside
make check.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -18,10 +18,12 @@ CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
     ON source_content(last_accessed_at);
 
 -- Processed variant blobs
--- Files stored at: cache/variants/<ab>/<cd>/<cache_key> (plus a .meta
--- sidecar with the content type). Tracked here (like source content
--- blobs above) so total cache usage is known without a directory scan,
--- and so LRU eviction has a timestamp to order on.
+-- Files stored at: cache/variants/<ab>/<cd>/<cache_key>. Tracked here
+-- (like source content blobs above) so total cache usage is known
+-- without a directory scan, and so LRU eviction has a timestamp to order
+-- on. A cache hit does not read this table: it takes the content type
+-- from the file's first bytes, and its last_accessed_at is written
+-- later, in a batch.
 CREATE TABLE IF NOT EXISTS variant_content (
     cache_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,