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>

examples/library/recover.ludic

@@ -0,0 +1,37 @@
+# recover.ludic — recoverable failures as values (issue #46). A fallible function
+# returns a `result`: `ok(payload)` on success, `err(message)` on failure. The
+# caller recovers a value with `try EXPR else { … }` — on `ok` the expression is
+# the payload, on `err` the else block runs (with the message bound to `error`)
+# and its trailing expression is the fallback. No exceptions, no unwinding — just
+# a branch on the result's tag. `is_ok`/`is_err` classify without unwrapping.
+#
+# This is the SUCCESS-style showcase (it always runs to the end), printing:
+#   2600 100 1 1 42 7
+program Recover {
+  # loading a possibly-missing save: slot 1 has data, everything else is empty.
+  function load_score(slot: int) -> result {
+    if slot == 1 { return ok(2600) }
+    return err("no save in slot")
+  }
+
+  entry {
+    # ok path: the payload flows straight out of `try`
+    print(try load_score(1) else { 0 })       # 2600
+
+    # err path: the else block supplies a fallback (a fresh game)
+    print(try load_score(4) else { 100 })     # 100
+
+    # the else block can read the failure message via `error`, and log/branch
+    var handled = 0
+    let s = try load_score(9) else {
+      if error == "no save in slot" { handled = 1 }
+      100                                       # trailing expression = fallback
+    }
+    print(handled)                             # 1
+    print(s / 100)                             # 1  (fallback was 100)
+
+    # is_ok / is_err classify a result without unwrapping it
+    if is_ok(load_score(1)) { print(42) }      # 42
+    if is_err(load_score(2)) { print(7) }      # 7
+  }
+}