harness: withdraw five pre-code ADRs to deferred ideas

Twenty-seven ADRs existed before a line of Go. Five specified the shape of
unbuilt mechanisms — cache validity, declared types, the settings cascade,
taxonomies, extras — which breaks the rule against abstraction before a
second concrete use, in prose where the counters cannot see it. They move to
ideas/deferred-decisions.md as recorded intent and return shaped by whatever
implements them.

Citations retargeted throughout; where one was decoration the rule now stands
on its own reasoning. Type declarations and the cascade drop to [spec] with
the MVP behaviour stated instead, so the first prompts have less to build.
conventions.md names http.ServeMux as the router, closing a hole that invited
hand-rolling a path splitter. The ADR gate now checks a number is registered
in the log rather than headed by an entry, so withdrawals resolve and invented
numbers still fail. Two architecture invariants corrected: identity no longer
implies a required language suffix, and the duplicated permalink clause is gone.

docs/roadmap.md

@@ -70,3 +70,4 @@ Bengali tokenisation for search is the genuinely novel problem here — solve it
 and deliberately, not as a side effect of adding search. The low-bandwidth, low-carbon ethos is
 coherent with Gemini output, PWA offline, and no-JS defaults; let that coherence break ties when two
 designs are otherwise equal.
+