-- default-on-err r d — unwrap R T E to T, returning d if Err.
--
-- The Result mirror of `??` (nil-coalesce for O T). Use when you have a
-- Result from num, rd, get, env, etc. and want a safe fallback without
-- writing a full match arm:
--
-- v = default-on-err (num s) 0 -- 0 if s is not a number
--
-- Prefer over `?r{~v:v;^_:d}` when the error payload is unused.
-- Verifier checks that d matches the Ok type (ILO-T040 on mismatch).
-- Using `??` on a Result triggers ILO-T041 with a hint pointing here.
-- Ok path: num "42" succeeds, inner value returned.
ok-num>n;default-on-err (num "42") 0
-- Err path: num "bad" fails, default 0 returned.
err-num>n;default-on-err (num "bad") 0
-- Ok path with text type.
ok-text>t;r=~"hello";default-on-err r "fallback"
-- Err path with text type.
err-text>t;r=^"oops";default-on-err r "fallback"
-- Chained: two calls, results used in arithmetic.
chain>n;a=default-on-err (num "10") 0;b=default-on-err (num "bad") 0;+ a b
-- run: ok-num
-- out: 42
-- run: err-num
-- out: 0
-- run: ok-text
-- out: hello
-- run: err-text
-- out: fallback
-- run: chain
-- out: 10