review quick-wins: JIT gate, crash guards, crit-2 signal-return, regen repair

Server (sx_server.ml):
- HTTP mode: JIT hook now opt-in via SX_SERVING_JIT, matching epoch mode
  (was unconditional — live serving-JIT miscompiles J1/J2/J3 de-risked)
- command channel: malformed/non-ASCII line returns an error response
  instead of killing the shared process (C1/C1b)
- response cache: soft error pages no longer cached (S4);
  http_render_page returns (html, is_error)

Kernel spec + regen:
- crit-2: signal-return frame stored the saved kont under :f but the reader
  looked up "saved-kont" — handler value became the whole program's result
  and the covering test passed vacuously. Fixed; raise-continuable now also
  resumes at the raise site (rest-k, not unwound-k), mirroring signal-condition
- quasiquote: R7RS longhand unquote-splicing aliased to splice-unquote
  (used to serialize literally — silent zero-splice)
- guard: re-raise sentinel gensym'd per execution (was forgeable by any
  (list '__guard-reraise__ x) value)
- do: IIFE-head form no longer misparses as a Scheme do-loop
- render: area/base/embed/param/track added to HTML_TAGS (were void-only
  and rendered as Undefined symbol)
- REGEN REPAIR: checked-in sx_ref.ml carried hand-written additions that
  every regeneration silently lost (let-values/define-values/delay/
  delay-force registrations, AdtValue define-type) plus 5 regen blockers
  (arrow-name mangling, 3-arg get, &rest defines, HO-position helper refs,
  transpiler prim-table gaps). Moved into bootstrap.py FIXUPS/skips and the
  transpiler prim table — regen is now reproducible, compiles, and tests
  at baseline (CI Dockerfile.test steps 3-4 could not previously have
  produced a compiling kernel)

Primitives:
- contains?: dict key-check arm per its spec doc
- expt: promotes to float on int63 overflow ((expt 2 100) returned 0)
- mcp_tree parity with sx_primitives: get (Integer indices + 3-arg default),
  split (literal substring, was char-class — the historical gotcha lived
  here), empty? on ""/{}, contains?, equal?, keyword-name, char-code
  (Integer), parse-number (Integer-aware)

Python/docs:
- shared/sx/boundary.py: dead validation now logs a one-time WARNING instead
  of silently no-oping (full revival gated: tier-1 declarations deleted and
  SX_BOUNDARY_STRICT=1 is live in production compose)
- CLAUDE.md: canonical reference now points at spec/*.sx; island authoring
  rules corrected (let IS sequential, bodies ARE implicit begin)

Verification: full suite 5762 passed / 274 failed — fail set byte-identical
to the pre-change baseline (273 in-progress hs-* + pre-existing r7rs radix
shadow). All repros verified fixed on both the native binary and the rebuilt
WASM browser kernel. Review findings: /tmp/sx-review/*.md

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["unix-100664f4",[2]],["sx-809891af",[2,3,4]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,6]],["dune__exe__Sx_browser-4878f9e1",[2,5,7]],["std_exit-10fb8830",[2]],["start-8c705527",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["unix-100664f4",[2]],["sx-c1aa02f5",[2,3,4]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,6]],["dune__exe__Sx_browser-b2ad0885",[2,5,7]],["std_exit-10fb8830",[2]],["start-8c705527",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new