Chez Phase 3 inc9a: self-contained Chez bootstrap (no Janet in the loop)

Makes the inc8 fixpoint the actual build. host/chez/bootstrap.ss loads a seed
(prelude, image) pair and rebuilds the clojure.core prelude + compiler image from
source via the on-Chez compiler — read/analyze/emit all on Chez, zero Janet.

The seed (host/chez/seed/{prelude,image}.ss) is the checked-in bootstrap
compiler, minted once via the fixpoint (driver/mint-chez-seed* iterates
bootstrap.ss from the Janet-emitted pair to a joint byte-fixpoint). It's a joint
fixpoint: rebuilding from an up-to-date seed reproduces it exactly. So a fresh
checkout + Chez (no Janet) yields a working jolt.

test/chez/bootstrap-test.janet spawns only chez, asserts the rebuilt artifacts
match the seed byte-for-byte and compile+run real cases. Drift (seed sources
changed) fails the test with a re-mint pointer; host/chez/seed/README documents
re-minting.

host/chez/seed/README.md

@@ -0,0 +1,39 @@
+# 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:
+
+- `prelude.ss` — the `clojure.core` prelude (all tiers + clojure.string/walk/
+  template/edn/set/pprint) as Scheme `def-var!` forms.
+- `image.ss` — the compiler image (`jolt.ir` + `jolt.analyzer` +
+  `jolt.backend-scheme`) as Scheme `def-var!` forms.
+
+Both are **generated**, not hand-written. They are checked in because a fresh
+checkout must be able to build jolt-on-Chez using only Chez: `host/chez/bootstrap.ss`
+loads this seed, then rebuilds the prelude + image from the `.clj`/`.ss` sources via
+the on-Chez compiler (read → analyze → emit, all on Chez). The seed is a **joint
+byte-fixpoint**: rebuilding from an up-to-date seed reproduces it exactly
+(`test/chez/bootstrap-test.janet` verifies this).
+
+Janet was used once, historically, to mint the very first seed (the Janet analyzer/
+emitter cross-compiled the sources, then the on-Chez compiler iterated to the
+fixpoint — see `test/chez/fixpoint-test.janet`). After that, Janet is never needed
+to build or run jolt-on-Chez.
+
+## Re-minting
+
+When the seed sources change (the core tiers, the compiler namespaces, the host
+contract, the reader, `emit-image.ss`), the seed drifts and `bootstrap-test`
+fails. Re-mint it:
+
+```janet
+(import host/chez/driver :as d)
+(import host/chez/jolt-chez :as jc)
+(def ctx (d/make-ctx))
+(d/mint-chez-seed* (jc/ensure-prelude ctx)
+                   (d/ensure-compiler-image ctx "/tmp/stage1.ss")
+                   "host/chez/seed/prelude.ss"
+                   "host/chez/seed/image.ss")
+```
+
+Then commit the refreshed `prelude.ss` / `image.ss`.