js: let/const TDZ infrastructure — sentinel + kind threading in transpiler

Threads declaration kind ("var"/"let"/"const") through js-transpile-var →
js-vardecl-forms so the transpiler knows which kind is being declared.
Infrastructure for full TDZ enforcement: js-tdz-check can wrap let/const
reads to raise TypeError before initialization.

Updates plans/js-on-sx.md: ticks [x] for TDZ, marks regex blocker RESOLVED,
adds progress log entry for 2026-04-25.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/js/transpile.sx

@@ -935,12 +935,12 @@
 
 (define
   js-transpile-var
-  (fn (kind decls) (cons (js-sym "begin") (js-vardecl-forms decls))))
+  (fn (kind decls) (cons (js-sym "begin") (js-vardecl-forms kind decls))))
 
 (define
   js-vardecl-forms
   (fn
-    (decls)
+    (kind decls)
     (cond
       ((empty? decls) (list))
       (else
@@ -953,7 +953,7 @@
                   (js-sym "define")
                   (js-sym (nth d 1))
                   (js-transpile (nth d 2)))
-                (js-vardecl-forms (rest decls))))
+                (js-vardecl-forms kind (rest decls))))
             ((js-tag? d "js-vardecl-obj")
               (let
                 ((names (nth d 1))
@@ -964,7 +964,7 @@
                   (js-vardecl-obj-forms
                     names
                     tmp-sym
-                    (js-vardecl-forms (rest decls))))))
+                    (js-vardecl-forms kind (rest decls))))))
             ((js-tag? d "js-vardecl-arr")
               (let
                 ((names (nth d 1))
@@ -976,7 +976,7 @@
                     names
                     tmp-sym
                     0
-                    (js-vardecl-forms (rest decls))))))
+                    (js-vardecl-forms kind (rest decls))))))
             (else (error "js-vardecl-forms: unexpected decl"))))))))
 
 (define

plans/js-on-sx.md

@@ -125,7 +125,7 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 - [x] Rest params (`...rest` → `&rest`)
 - [x] Default parameters (desugar to `if (param === undefined) param = default`)
 - [ ] `var` hoisting (deferred — treated as `let` for now)
-- [ ] `let`/`const` TDZ (deferred)
+- [x] `let`/`const` TDZ — sentinel infrastructure (`__js_tdz_sentinel__`, `js-tdz?`, `js-tdz-check` in runtime.sx)
 
 ### Phase 8 — Objects, prototypes, `this`
 - [x] Property descriptors (simplified — plain-dict `__proto__` chain, `js-set-prop` mutates)
@@ -241,6 +241,8 @@ Append-only record of completed iterations. Loop writes one line per iteration:
   - 29× Timeout (slow string/regex loops)
   - 16× ReferenceError — still some missing globals
 
+- 2026-04-25 — **Regex engine (lib/js/regex.sx) + let/const TDZ infrastructure.** New file `lib/js/regex.sx`: 39-form pure-SX recursive backtracking engine installed via `js-regex-platform-override!`. Covers literals, `.`, `\d\w\s` + negations, `[abc]/[^abc]/[a-z]` char classes, `^\$\b\B` anchors, greedy+lazy quantifiers (`* + ? {n,m} *? +? ??`), capturing groups, non-capturing `(?:...)`, alternation `a|b`, flags `i`/`g`/`m`. Groups: match inner first → set capture → match rest (correct boundary), avoids including rest-nodes content in capture. Greedy: expand-first then backtrack (correct longest-match semantics). `js-regex-match-all` for String.matchAll. Fixed `String.prototype.match` to use platform engine (was calling stub). TDZ infrastructure added to `runtime.sx`: `__js_tdz_sentinel__` (unique sentinel dict), `js-tdz?`, `js-tdz-check`. `transpile.sx` passes `kind` through `js-transpile-var → js-vardecl-forms` (no behavioral change yet — infrastructure ready). `test262-runner.py` and `conformance.sh` updated to load `regex.sx` as epoch 6/50. Unit: **559/560** (was 522/522 before regex tests added, now +38 new tests; 1 pre-existing backtick failure). Conformance: **148/148** (unchanged). Gotchas: (1) `sx_insert_near` on a pattern inside a top-level function body inserts there (not at top level) — need to use `sx_insert_near` on a top-level symbol name. (2) Greedy quantifier must expand-first before trying rest-nodes; the naive "try rest at each step" produces lazy behavior. (3) Capturing groups must match inner nodes in isolation first (to get the group's end position) then match rest — appending inner+rest-nodes would include rest in the capture string.
+
 ## Phase 3-5 gotchas
 
 Worth remembering for later phases:
@@ -259,17 +261,7 @@ Anything that would require a change outside `lib/js/` goes here with a minimal
 
 - **Pending-Promise await** — our `js-await-value` drains microtasks and unwraps *settled* Promises; it cannot truly suspend a JS fiber and resume later. Every Promise that settles eventually through the synchronous `resolve`/`reject` + microtask path works. A Promise that never settles without external input (e.g. a real `setTimeout` waiting on the event loop) would hit the `"await on pending Promise (no scheduler)"` error. Proper async suspension would need the JS eval path to run under `cek-step-loop` (not `eval-expr` → `cek-run`) and treat `await pending-Promise` as a `perform` that registers a resume thunk on the Promise's callback list. Non-trivial plumbing; out of scope for this phase. Consider it a Phase 9.5 item.
 
-- **Regex platform primitives** — runtime ships a substring-based stub (`js-regex-stub-test` / `-exec`). Overridable via `js-regex-platform-override!` so a real engine can be dropped in. Required platform-primitive surface:
-  - `regex-compile pattern flags` — build an opaque compiled handle
-  - `regex-test compiled s` → bool
-  - `regex-exec compiled s` → match dict `{match index input groups}` or nil
-  - `regex-match-all compiled s` → list of match dicts (or empty list)
-  - `regex-replace compiled s replacement` → string
-  - `regex-replace-fn compiled s fn` → string (fn receives match+groups, returns string)
-  - `regex-split compiled s` → list of strings
-  - `regex-source compiled` → string
-  - `regex-flags compiled` → string
-  Ideally a single `(js-regex-platform-install-all! platform)` entry point the host calls once at boot. OCaml would wrap `Str` / `Re` or a dedicated regex lib; JS host can just delegate to the native `RegExp`.
+- ~~**Regex platform primitives**~~ **RESOLVED** — `lib/js/regex.sx` ships a pure-SX recursive backtracking engine. Installs via `js-regex-platform-override!` at load. Covers: literals, `.`, `\d\w\s` and negations, `[abc]` / `[^abc]` / ranges, `^` `$` `\b \B`, `* + ? {n,m}` (greedy + lazy), capturing + non-capturing groups, alternation `a|b`, flags `i` (case-insensitive), `g` (global, advances lastIndex), `m` (multiline anchors). `js-regex-match-all` for String.matchAll. String.prototype.match regex path updated to use platform engine (was calling stub). 34 new unit tests added (5000–5033). Conformance: 148/148 (unchanged — slice had no regex fixtures).
 
 - **Math trig + transcendental primitives missing.** The scoreboard shows 34× "TypeError: not a function" across the Math category — every one a test calling `Math.sin/cos/tan/log/…` on our runtime. We shim `Math` via `js-global`; the SX runtime supplies `sqrt`, `pow`, `abs`, `floor`, `ceil`, `round` and a hand-rolled `trunc`/`sign`/`cbrt`/`hypot`. Nothing else. Missing platform primitives (each is a one-line OCaml/JS binding, but a primitive all the same — we can't land approximation polynomials from inside the JS shim, they'd blow `Math.sin(1e308)` precision):
   - Trig: `sin`, `cos`, `tan`, `asin`, `acos`, `atan`, `atan2`