feat(errors): recoverable failures as values — try/else over ok/err results (#46)

A fallible function returns a `result` value, built with ok(payload) on success
or err(message) on failure. The caller recovers a value with `try EXPR else {
… }`: on ok the whole expression is the payload; on err the else block runs —
with the failure message bound to `error` — and its trailing expression supplies
the fallback. It is a plain branch on the result's tag: no exceptions, no hidden
control flow, nothing unwinds. is_ok(r) / is_err(r) classify without unwrapping.

Payloads are any i32-width scalar (int/fixed/bool/entity). The feature is
additive and only kicks in when ok/err/try are used, so untouched programs
compile byte-identically (verified) and the C-free bootstrap fixpoint holds.
Complements panic/assert from #8 (the unrecoverable half). The optional
top-level frame `recover` stays deferred (needs a frame-abort mechanism); the
full tagged-union/any generalization is tracked in #1.

Adds the `try` keyword and ok/err/is_ok/is_err builtins across the compiler,
the vocabulary header, JetBrains + TextMate/VSCode grammars, the docs inventory
and pages, examples/library/recover.ludic, and a regression case.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/builtins/fn-ok.md

@@ -0,0 +1,30 @@
+---
+id: fn-ok
+name: ok
+category: builtins
+kind: builtin
+tokens: ok
+sig: ok(v) -> result
+tip: Wrap a success payload in a result — the happy half of try/else.
+order: 12
+---
+
+Wraps a success value in a <code>result</code> — the value type a fallible function returns. A <code>result</code> carries either a success payload (this) or a failure message (from <a href="fn-err"><code>err</code></a>), and a caller recovers it with <a href="kw-try"><code>try</code></a>/<code>else</code> or classifies it with <a href="fn-is_ok"><code>is_ok</code></a> / <a href="fn-is_err"><code>is_err</code></a>. A function that can fail declares <code>-&gt; result</code> and returns <code>ok(value)</code> on the happy path.
+
+The payload is any <code>i32</code>-width scalar — <code>int</code>, <code>fixed</code>, <code>bool</code>, or <code>entity</code>.
+
+Parameters:
+- `v` — the success payload
+
+```ludic
+program Parse {
+  function to_digit(c: int) -> result {
+    if c >= 48 and c <= 57 { return ok(c - 48) }   # '0'..'9'
+    return err("not a digit")
+  }
+  test "ok carries the parsed value" {
+    let d = try to_digit(55) else { 0 }            # '7' -> 7
+    expect_eq(d, 7)
+  }
+}
+```