split the content package at its seam

content.go had passed FILE_LOC_WARN, which conventions.md treats as the moment to
split rather than a number to ignore: flat until the figure, then split, never
pre-partitioned. content.go now parses bundles and builds permalinks; site.go holds
the indexed site — lookup with language fallback, aliases, Query and Run. The tests
follow the same seam.

No behaviour change, and the test-coupling gate was right to demand the tests move:
its exemption covers comments and whitespace, not code relocated between files,
where an edit could hide.

internal/content/doc.go

@@ -0,0 +1,5 @@
+// Package content reads a site root into bundles. It knows the disk and nothing about HTTP.
+//
+// Every read goes through an [os.Root] (ADR-0031), so no path — from a filename or later from a
+// request — can escape the site root, even through a symlink.
+package content