fix(tiers): evaluate :or destructuring defaults eagerly in every tier

[csm]

Clojure's destructure expands a symbol carrying an :or entry to
(get m :k default). get is an ordinary function, so the default is
evaluated whether or not the key is present. The tree-walker does that;
the compiled tiers did not, and the two disagreed:

• lower_with_default lowered the default into the then arm of an
  IsNil branch, so it only ran on a miss. Lower it into the current
  block instead — straight-line and unconditional — and keep the branch
  purely as a select over two already-computed values.

• execute_ir bound the arity's destructuring patterns into the env via
  bind_fn_params on top of the prologue the lowerer emits, so with the
  eager lowering above each default would run twice per call. The ANF
  lowerer never emits LoadLocal, so those env bindings are never read:
  bind only the named params there (bind_fn_params_positional), and let
  the prologue own the destructuring.

Because tier-up happens partway through a run, and lowering is driven by a
background worker, the old split made a side-effecting or throwing default
fire a nondeterministic number of times: the issue's reproduction reported
133/130/140/130/144 evaluations out of 200 calls across five runs of the
same binary. It now reports 200 in every tier and every run.

Pinned in two places: execution_tier_parity gains an :or case with a
side-effect counter, so tree-walk, eager IR, JIT and AOT must agree on the
count (it fails without either half of this fix), and a new
destructure_or_default_eager test runs the discriminating programs
through the tree-walker and the IR tier side by side.

Fixes #363

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01NLQqFN41sWfs1hF5uyfN8N