Docs: Chez-only, drop the Janet-era references and obsolete migration notes

Bring the docs in line with the actual implementation now that Chez is the sole
substrate.

Deleted the migration/spike/handoff artifacts that only documented the Janet
era or the port effort: the port plan, phase-0 and foundational-runtime spike
writeups (+ the stray root-level copy), the self-hosting design notes, the
architecture-refactor plan, and spike/chez/RESULTS.md.

Rewrote the current reference docs against the Chez facts: building-and-deps and
tools-deps (no jpm/build step — bin/joltc off the checked-in seed, deps via
jolt.deps into ~/.jolt/gitlibs), libraries (SQLite is built-in jdbc.core over
libsqlite3, not a Janet driver), the conformance/spec test-flow docs (the Chez
corpus runner + certify, no .janet harnesses), and the transient / type-hint /
seed-overlay design notes (Chez representations: mutable transients, flat
copy-on-write vectors, HAMT maps, the seed/overlay twin). Fixed the README
collections line (vectors aren't 32-way tries) and added the ffi/transient gate
targets. rfc 0001's numerics open-question is resolved (the Scheme tower).

Renamed the built-in HTTP adapter to jolt.http.server only (dropped the
ring-janet.adapter alias — a Janet-era name).

docs/spec/README.md

@@ -38,10 +38,13 @@ Of the 694 `clojure.core` vars in the ClojureDocs inventory:
 
 ## How this connects to the test suites
 
-- `test/integration/conformance-test.janet` — 302 assertions, each run
-  through three independent execution paths (interpreter, bootstrap
-  compiler, self-hosted compiler) that must agree. Spec entries cite these.
-- `test/spec/*.janet` — ~1,500 behavioral cases organized by topic.
+- `test/chez/corpus.edn` — the host-neutral behavioral corpus, one row per
+  case (`{:suite :label :expected :actual}`). The Chez compiler evaluates each
+  case via `host/chez/run-corpus.ss` (run with `make corpus`), and
+  `test/conformance/certify.clj` certifies every `:expected` against reference
+  JVM Clojure (run with `make certify`). Spec entries cite these cases.
+- `test/conformance/` — the certification tooling and classified divergences
+  (`certify.clj`, `known-divergences.edn`); see its `README.md` and `SPEC.md`.
 - `vendor/clojure-test-suite` — the cross-dialect suite (≥4081 assertions
   passing); dialect splits there are classification evidence.
 - jank's per-construct corpus (`~/src/jank/compiler+runtime/test/jank`) is