publish a bundle's extras as a browsable tree

Re-adopts the parked extras entry as ADR-0047: `extras/` inside a bundle is skipped
by the scanner entirely, so a `.md` in there is an asset with no identity and no URL
of its own. The engine enumerates the tree, sorts it by path, classifies by
extension, renders markdown and text, and offers everything else as bytes. One route
with two behaviours — `…/extras/{path}` selects an entry, `?raw` returns the file.

Almost everything it needed already existed, which is the sign the model was right:
the scanner had a directory exclusion, `Assets()` knew which bundles own a
directory, and `Lookup` already decided visibility — so a draft hides its extras
with no new check. A test proves that, including `?raw`.

Two deviations from the parked shape, both because the shape was written before the
code. The directory name is fixed rather than a cascade key, since nothing reads a
section-level setting yet. And an entry is resolved against the *enumeration* rather
than the filesystem: not being in the listing is a stronger answer than os.Root
refusing a path, and cheaper.

Selecting is a link and a full page. No JavaScript is involved, and a
sidebar-and-pane layout is the theme's business — which is the layer rule applied
before writing the feature rather than after.

Three size warnings fired as a result and were fixed by splitting at seams, not by
sharding: render.go gave up its type declarations to view.go, which is the theme
contract in Go and nothing else; serve() split into a dispatcher and serveBundle;
resolve() gave up its language-prefix step to cutLang.

ideas/deferred-decisions.md

@@ -94,6 +94,10 @@ type declaration, so this and declared types land together.
 
 ## Extras
 
+Status: **adopted → ADR-0047**, with the shape discovered by building it. Two things changed on contact: the
+directory name is fixed rather than a cascade key (nothing reads a section-level setting yet), and an entry is
+resolved against the enumeration rather than the filesystem.
+
 Deferred because not in the MVP, and its shape should be discovered by building it.
 
 ## Extras: local assets, enumerated and browsable