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/backend/emit_decl.ludic

@@ -140,6 +140,7 @@ function emit_program() -> void {
   g_uses_loopback = false
   g_uses_expect = false
   g_uses_panic = false
+  g_uses_result = false
   g_cov_lines = new []int
   g_cov_active = true
   loc_name = new []pointer;  loc_reg = new []pointer;  loc_ty = new []pointer;  loc_mut = new []int
@@ -198,6 +199,7 @@ function emit_program() -> void {
     emith("declare i32 @fprintf(ptr, ptr, ...)\n")
     emith("@.fmt_panic = private unnamed_addr constant [6 x i8] c\"%s%s\\0A\\00\"\n")
   }
+  if g_uses_result { emith("%Result = type { i32, i32, ptr }\n") }   # issue #46: ok/err/try value
   emit_cov_runtime()                              # issue #45: --coverage tables + exit dump
 }