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-is_err.md

@@ -0,0 +1,28 @@
+---
+id: fn-is_err
+name: is_err
+category: builtins
+kind: builtin
+tokens: is_err
+sig: is_err(r: result) -> bool
+tip: True when a result carries a failure.
+order: 15
+---
+
+Tests a <code>result</code>'s tag: <code>true</code> when it was built with <a href="fn-err"><code>err</code></a>, <code>false</code> when it was built with <a href="fn-ok"><code>ok</code></a>. Use it to detect the failure case explicitly — for example to count or log failures — while <a href="kw-try"><code>try</code></a>/<code>else</code> handles recovering a value. It is the exact negation of <a href="fn-is_ok"><code>is_ok</code></a>.
+
+Parameters:
+- `r` — the result to test
+
+```ludic
+program Retry {
+  function step(n: int) -> result {
+    if n < 3 { return err("not ready") }
+    return ok(n)
+  }
+  test "is_err detects the failure case" {
+    expect(is_err(step(1)))
+    expect(not is_err(step(5)))
+  }
+}
+```