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).

host/chez/seed/README.md

@@ -1,7 +1,7 @@
 # Chez bootstrap seed
 
-These two files are the **bootstrap compiler** for jolt-on-Chez — the seed that
-makes the build self-hosting with no Janet in the loop:
+These two files are the **bootstrap compiler** for jolt — the seed that makes
+the build self-hosting:
 
 - `prelude.ss` — the `clojure.core` prelude (all tiers + clojure.string/walk/
   template/edn/set/pprint) as Scheme `def-var!` forms.