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>

tools/docgen/inventory.json

@@ -48,7 +48,11 @@
     "fn-read_char",
     "fn-file_write",
     "fn-file_stdout",
-    "fn-file_stderr"
+    "fn-file_stderr",
+    "fn-ok",
+    "fn-err",
+    "fn-is_ok",
+    "fn-is_err"
   ],
   "collision": [
     "collision-rects",
@@ -72,7 +76,8 @@
     "kw-match",
     "kw-machine",
     "kw-state",
-    "kw-become"
+    "kw-become",
+    "kw-try"
   ],
   "ease": [
     "ease-in",