feat(stdlib): close 6 STEP-3 Deno-ESM gaps surfaced by TS→AS ports (Refs #239, #242) (#504)

Closes the smallest, safest tractable gaps that the four merged STEP-2
TS→AffineScript ports — phronesis#19, session-sentinel#25,
tropical-resource-typing#15, nafa-app#23 — flagged for STEP 3 of the
estate-wide campaign (`hyperpolymath/standards#239`, sub-issue `#242`).

## What lands

### `stdlib/Deno.affine` (+6 externs)

| extern | lowers to | gap |
|---|---|---|
| `statIsFile(path) -> Bool` | `Deno.statSync(path).isFile` | 3 |
| `statIsDirectory(path) -> Bool` | `Deno.statSync(path).isDirectory` |
3 |
| `bytesLength(b) -> Int` | `(b).length` | 4 |
| `bytesByteAt(b, i) -> Int` | `(b)[i]` | 4 |
| `bytesAsciiSlice(b, s, e) -> String` |
`String.fromCharCode(...(b).slice(s, e))` | 4 |
| `importMetaUrl() -> String` | `import.meta.url` | 5 |

The `bytes*` accessors give AffineScript first-class read access to the
opaque `Bytes` returned by `readFileBytes` so callers can peek at
file-magic / fixed-width binary headers without round-tripping through
`readTextFile` (which throws on binary). `importMetaUrl` exposes the JS
`__dirname` / `__filename` idiom for "find my own location"; only legal
at module top level, which the Deno-ESM backend's output already is.

### `stdlib/string.affine` (1 char)

`fn ends_with` → `pub fn ends_with`. The function existed but was
private, so the four STEP-2 ports each inlined a `string_sub`-backed
helper instead of importing it. Gap 2.

### `lib/codegen_deno.ml` — wildcard `let _` fix (gap 7)

`StmtLet { sl_pat = PatWildcard _; … }` now lowers to a bare expression
statement instead of `const _ = X;`. Three back-to-back `let _ = side()`
discards in the same scope tripped JS `SyntaxError: Identifier '_' has
already been declared`. The wildcard pattern doesn't bind, so dropping
the binding entirely preserves AffineScript semantics (evaluate for side
effects).

```js
// before
function discard_chain() {
  const _ = side(1);
  const _ = side(2);   // ❌ SyntaxError
  const _ = side(3);
  return 42;
}

// after
function discard_chain() {
  side(1);
  side(2);
  side(3);
  return 42;
}
```

### Tests

`tests/codegen-deno/deno_scripting_part2.{affine,deno.js,harness.mjs}` —
11 assertions across the new lowerings + the wildcard fix. The harness
reaching its discard-chain assertion is itself the test for gap 7: a
syntax-error generated module would have thrown at the dynamic `import`
line before any assert ran.

## Verification

| step | result |
|---|---|
| `dune build bin/main.exe` | ✅ |
| `dune runtest` | ✅ 353/353 |
| `./tools/run_codegen_deno_tests.sh` | ✅ all harnesses (incl. the new
one) |

## What does not land here (deferred deliberately)

- **gap 1** — `Deno.test` extern lowering. The four merged STEP-2 ports
use a panic-on-fail `main()` driver and it works; not unblocking STEP 4.
- **gap 6** — native TOML parser. Heavier work; STEP-2 ports used regex
field-presence checks where TOML mattered.

Gaps 9 (`AFFINESCRIPT_STDLIB` env discovery ladder, shipped #433) and 10
(negative integer literals, verified working in all contexts — match
arms, array literals, equality, ternaries) were audited as
already-resolved.

## Refs

- Closes 6 of the 10 gaps blocking `hyperpolymath/standards#242` (STEP 3
of `#239`).
- Unblocks STEP 4 (`#243`, mid-tier 4-9 file ports) and STEP 5 (`#244`,
idaptik DLC) for any consumer that needs stat predicates, byte
accessors, module-URL inspection, the public `ends_with`, or multiple
`let _` discards in a single scope.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

lib/codegen_deno.ml

@@ -513,6 +513,14 @@ let () =
   b "ensureDir"     (fun a -> Printf.sprintf "__as_ensureDir(%s)" (arg 0 a));
   b "readDirNames"  (fun a -> Printf.sprintf "__as_readDirNames(%s)" (arg 0 a));
   b "statSize"      (fun a -> Printf.sprintf "Deno.statSync(%s).size" (arg 0 a));
+  b "statIsFile"      (fun a -> Printf.sprintf "Deno.statSync(%s).isFile" (arg 0 a));
+  b "statIsDirectory" (fun a -> Printf.sprintf "Deno.statSync(%s).isDirectory" (arg 0 a));
+  b "bytesLength"     (fun a -> Printf.sprintf "(%s).length" (arg 0 a));
+  b "bytesByteAt"     (fun a -> Printf.sprintf "(%s)[%s]" (arg 0 a) (arg 1 a));
+  b "bytesAsciiSlice" (fun a -> Printf.sprintf "String.fromCharCode(...(%s).slice(%s, %s))" (arg 0 a) (arg 1 a) (arg 2 a));
+  (* `import.meta.url` — only legal at module top level, which the
+     Deno-ESM backend's output already is. *)
+  b "importMetaUrl"   (fun _ -> "import.meta.url");
   b "pathJoin"      (fun a -> Printf.sprintf "__as_pathJoin(%s, %s)" (arg 0 a) (arg 1 a));
   b "isNotFound"    (fun a -> Printf.sprintf "__as_isNotFound(%s)" (arg 0 a));
   (* ---- JSON ---- *)
@@ -1251,6 +1259,13 @@ and gen_try_stmt ctx body catch finally =
 
 and gen_stmt ctx (stmt : stmt) : string =
   match stmt with
+  | StmtLet { sl_pat = PatWildcard _; sl_value; _ } ->
+      (* `let _ = X` evaluates X for side effects and drops the value.
+         Emitting `const _ = X;` produces a JS SyntaxError on the second
+         occurrence in the same scope ("Identifier '_' has already been
+         declared"). Drop the binding entirely; the bare expression
+         statement keeps the semantics. *)
+      gen_expr ctx sl_value ^ ";"
   | StmtLet { sl_pat; sl_value; sl_mut; sl_quantity = _; sl_ty = _ } ->
       let kw = if sl_mut then "let" else "const" in
       let js = kw ^ " " ^ gen_pattern ctx sl_pat ^ " = "

stdlib/Deno.affine

@@ -68,6 +68,14 @@ pub extern fn readDirNames(path: String) -> [String];
 /// `Deno.statSync(path).size` in bytes.
 pub extern fn statSize(path: String) -> Int;
 
+/// `Deno.statSync(path).isFile` — true if `path` is a regular file.
+/// Throws on a missing path (pair with `isNotFound` for the absent case).
+pub extern fn statIsFile(path: String) -> Bool;
+
+/// `Deno.statSync(path).isDirectory` — true if `path` is a directory.
+/// Throws on a missing path (pair with `isNotFound` for the absent case).
+pub extern fn statIsDirectory(path: String) -> Bool;
+
 /// Recursive walk under `root` — every file path beneath it, depth-first.
 /// Mirrors `std/fs/walk` for the common case (no glob filter; callers
 /// filter by extension). Throws on a missing root via `Deno.readDirSync`.
@@ -164,6 +172,15 @@ pub extern fn dateNow() -> Int;
 /// returns epoch millis as `Int`.
 pub extern fn dateNowIso() -> String;
 
+// ── Module identity ────────────────────────────────────────────────
+
+/// `import.meta.url` — the absolute URL of the importing module. The JS
+/// idiom for "find my own location" (cf. `__dirname` / `__filename`). At
+/// Deno-ESM top level, lowers to the bare `import.meta.url` expression;
+/// callers parse it (`new URL(...)`/`fileURLToPath`/string split) for
+/// directory-relative behaviour.
+pub extern fn importMetaUrl() -> String;
+
 // ── CLI ────────────────────────────────────────────────────────────
 
 /// `Deno.args` — command-line arguments (excludes argv[0]).

tests/codegen-deno/deno_scripting_part2.affine

@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: MPL-2.0
+// campaign #239 STEP 3 — second batch of Deno-scripting stdlib gaps
+// surfaced during STEP 2 ports (panic-attack#82, session-sentinel#25,
+// tropical-resource-typing#15, nafa-app#23).
+//
+// New externs / fixes exercised:
+//   - statIsFile / statIsDirectory         (gap 3)
+//   - bytesLength / bytesByteAt /
+//     bytesAsciiSlice                      (gap 4)
+//   - importMetaUrl                        (gap 5)
+//   - let _ = X wildcard binding emits
+//     a bare expression statement          (gap 7)
+//
+// Signatures stay in primitives so the harness can stub the host.
+
+use Deno::{ statIsFile, statIsDirectory, bytesLength, bytesByteAt, bytesAsciiSlice, importMetaUrl, readFileBytes };
+
+pub fn classify_path(p: String) -> Int {
+  if statIsFile(p) {
+    1
+  } else {
+    if statIsDirectory(p) {
+      2
+    } else {
+      0
+    }
+  }
+}
+
+pub fn first_byte(p: String) -> Int {
+  let b = readFileBytes(p);
+  if bytesLength(b) == 0 {
+    0
+  } else {
+    bytesByteAt(b, 0)
+  }
+}
+
+pub fn header_string(p: String, n: Int) -> String {
+  let b = readFileBytes(p);
+  let lim = bytesLength(b);
+  let end = if n > lim { lim } else { n };
+  bytesAsciiSlice(b, 0, end)
+}
+
+pub fn module_url_has_scheme() -> Bool {
+  let u = importMetaUrl();
+  // Any well-formed module URL starts with `file:` or `http`.
+  len(u) > 4
+}
+
+fn side(n: Int) -> Int { n + 1 }
+
+pub fn discard_chain() -> Int {
+  // Three back-to-back `let _ = side(N)` discards. Before the fix this
+  // tripped JS `SyntaxError: Identifier '_' has already been declared`
+  // because the AS pattern lowered to `const _ = ...` thrice. Now they
+  // lower to bare expression statements.
+  let _ = side(1);
+  let _ = side(2);
+  let _ = side(3);
+  42
+}