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>

selfhost/frontend/parse.ludic

@@ -150,6 +150,19 @@ function p_primary() -> Node {
     if (t.text == "false") { let n = node(E_BOOL); n.ival = 0; pi = pi + 1; return n }
     if (t.text == "null")  { pi = pi + 1; return node(E_NULL) }
     if (t.text == "new")   { pi = pi + 1;  let n = node(E_NEW); n.s = ptype();  if is_op("{") { n.a = record() };  return n }
+    # try EXPR else { ... } — evaluate a fallible (result-typed) expression; on
+    # `ok` the whole expression is its payload, on `err` the else block runs (with
+    # the message bound to `error`) and its trailing expression is the fallback.
+    if (t.text == "try") {
+      pi = pi + 1
+      let n = node(E_TRY); n.line = t.line
+      n.a = expr()
+      skipnl()
+      if not is_id("else") { perr("try needs an `else { ... }` fallback") }
+      pi = pi + 1; skipnl()
+      n.b = block()
+      return n
+    }
     let n = node(E_ID); n.s = t.text; pi = pi + 1; return n
   }
   if is_op("(") { pi = pi + 1; skipnl(); let e = expr(); skipnl(); eat_op(")"); return e }