sx: step 9 — parser feature registry

Add `_hs-feature-registry` dict and `hs-register-feature!` to
`lib/hyperscript/parser.sx`. Replace `parse-feat`'s hardcoded `cond`
on feature names with a registry lookup; the paren-open and
default-expression branches remain as fallthroughs.

Each parse-fn receives a `ctx` dict (built per call by `parse-feat-ctx`)
exposing parser internals (`:adv!`, `:tp-val`, `:tp-type`, `:at-end?`,
`:parse-cmd-list`, `:parse-expr`) and the per-feature handlers
(`:parse-on-feat` … `:parse-socket-feat`). All nine builtins
(`on`, `init`, `def`, `behavior`, `live`, `when`, `worker`, `bind`,
`socket`) are registered at file load time, so plugins added later via
`hs-register-feature!` persist across `hs-parse` calls.

Worker stub still raises identically. Mirror `shared/static/wasm/sx/hs-parser.sx`
copied byte-identical. OCaml: 4545/1339, JS: 2591/2465 — both match
baseline, zero regressions.

First piece of plans/designs/hs-plugin-system.md (Steps 10/11 follow).

plans/sx-improvements.md

@@ -215,7 +215,7 @@ these when operands are known numbers/lists.
 | 6 — JS AdtValue + define-type + match | [x] | fc8a3916 |
 | 7 — nested patterns | [x] | 0679edf5 |
 | 8 — exhaustiveness warnings | [x] | 6d391119 |
-| 9 — parser feature registry | [ ] | — |
+| 9 — parser feature registry | [x] | PENDING |
 | 10 — compiler + as converter registry | [ ] | — |
 | 11 — plugin migration + worker | [ ] | — |
 | 12 — frame records | [ ] | — |