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/extensions.md

@@ -58,7 +58,7 @@ wearing a disguise.
 | `PhasePage` | the assembled page object | OpenGraph, JSON-LD, related posts, series nav |
 | `PhaseOutput` | the final byte stream | minification, dithering, gemtext conversion |
 
-Every Stage runs on every bundle unless the cascade disables it (ADR-0017), and declares its trust
+Every Stage runs on every bundle unless a cascade key disables it (`ideas/deferred-decisions.md`), and declares its trust
 requirement. A Stage evaluating templates or shortcodes runs in trusted mode only, and the pipeline
 refuses it otherwise — enforced in code, not by convention, and
 tested with an untrusted-input case.