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).

test/conformance/SPEC.md

@@ -70,7 +70,6 @@ implements. Current profile (≈2670 portable, ≈249 non-portable):
 | `:concurrency/snapshot` | isolated-heap futures/agents/pmap — captured atoms are snapshotted, not shared |
 | `:host/jvm-interop` | Java classes / `instance?` on host classes / proxy / bean / definterface |
 | `:host/arrays` | Java arrays (`into-array`, `int-array`, …) |
-| `:host/janet` | Janet host interop (`janet.*`) |
 | `:async/core-async` | `clojure.core.async` channels/`go` |
 | `:runtime/eval` | runtime `eval` / `load-string` |
 | `:reader/jolt` | jolt reader features (`#?(:jolt …)`) + syntax-quote literal collapse |