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/00-front-matter.md

@@ -68,8 +68,9 @@ Behavioral questions are settled in this order: differential testing against
 the reference implementation → cross-dialect agreement in clojure-test-suite
 → ClojureDocs community examples (verified before inclusion) → reference
 source (for intent). Conformance tests live in this repository
-(`test/integration/conformance-test.janet` runs each assertion through three
-independent execution paths) and in the cross-dialect clojure-test-suite.
+(the corpus `test/chez/corpus.edn`, run on Chez via `host/chez/run-corpus.ss`
+and certified against reference JVM Clojure by `test/conformance/certify.clj`)
+and in the cross-dialect clojure-test-suite.
 
 ## 5. Chapter plan