fix(destructure): apply :or on absence, not on falsiness

[BuddhiLW]

Closes CLJEL-COMPILER map-destructuring-:or (20260710102141-320e86ab).
Fourth in the stack (#26 → #27 → #28 → this); bases retarget as each merges.

The bug

Clojure applies a destructuring default only when the key is absent. cljel
emitted (or (get m :x) 5), so a present falsy value was overridden:

(let [{:keys [x] :or {x 5}} {:x nil}] x)
;; => 5      wrong; Clojure gives nil

All four binding forms were affected: :keys, :strs, :syms and explicit.

The runtime half

Repointing the emitter at get's 3-arity is not enough on its own. clel-get
only distinguished absence from falsiness for hash tables — its list and
alist branches used the same or, so the bug would have survived for
alist-backed maps. Both branches now check presence, and index lookups tell a
nil element from an out-of-range index.

A second bug on the same line

The default was read with (get or-map sym), which cannot tell "no default"
from "the default is nil or false". :or {x false} was therefore dropped
entirely. It now uses contains?.

Structure

The four binding forms each carried their own copy of the default logic —
four places to get this right, and the reason one line had two bugs. They now
share one lookup, so the next change to default semantics happens once.

Evidence

Both new ERT tests fail against the previous runtime and pass against this
one. A compiled .cljel exercising all three binding forms was run in batch
Emacs:

present-nil   nil (want nil)     absent      5   (want 5)
present-val   3   (want 3)       strs nil    nil (want nil)
strs absent   1   (want 1)       expl nil    nil (want nil)
expl absent   2   (want 2)

610 Clojure tests, 3071 assertions, 0 failures. 30 ERT tests, 0 unexpected.