Fix stepper: lake SSR preservation + stack rebuild after stepping

Three fixes:

1. Framework: render-dom-lake preserves SSR elements during hydration.
   When client-side render-to-dom encounters a lake with an existing
   DOM element (from SSR), it reuses that element instead of creating
   a new one. This prevents the SSR HTML from being replaced with
   unresolvable raw SX expressions (~tw calls).

2. Stepper: skip rebuild-preview on initial hydration. Uses a non-
   reactive dict flag (not a signal) to avoid triggering the effect
   twice. On first run, just initializes the DOM stack from the
   existing SSR content by computing open-element depth from step
   types and walking lastElementChild.

3. Stepper: rebuild-preview computes correct DOM stack after re-render.
   Same depth computation + DOM walk approach. This fixes the bug where
   do-step after do-back would append elements to the wrong parent
   (e.g. "sx" span outside h1).

Also: increased code view font-size from 0.5rem to 0.85rem.

Playwright tests:
- lake never shows raw SX during hydration (mutation observer)
- back 6 + forward 6 keeps all 4 spans inside h1

Co-Authored-By: Claude Opus 4.6 (1M context) <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",[]],["sx-7cc5edb6",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-9ecd0d53",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-69cfbc7f",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-9ecd0d53",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",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