test(vcr-ra): non-vacuous --relocatable gate for const-CSE — proves it fires + stays correct on gale's path (#242)

[avrabe]

What

Closes the honesty gap in the const-CSE no-regression fix (#519): its --relocatable
gate was inert, so it gave zero positive evidence on the exact path gale's
gust_mix 90→92 B regression lived on.

--relocatable routes through select_direct(), where the optimized path's inline
const cache never runs — so the post-hoc apply_const_cse acts alone. But the
const_cse.wat arithmetic corpus never makes the direct selector emit the redundant
same-value-in-two-registers materialization that pass dedups, so the direct-path gate
was a tripwire that passed because nothing fired.

Change (test-only, behavior-frozen)

Adds const_cse_direct.wat — single-param, pure-register, reloc-free shapes (a

8-bit const reused across several independent sub-expressions summed at the end) that
do make the direct selector emit the redundant movw, so post-hoc apply_const_cse
fires on --relocatable: r1 44→40, r2 38→34.

The differential's direct-path gate is now non-vacuous — it asserts:

• (a) CSE actually fires on ≥1 function (fails if the gate goes blind);
• (b) no function grows (the no-regression property on gale's path);
• (c) every result is bit-identical to wasmtime under unicorn (correctness of
  post-hoc CSE on the direct selector's output).

This is the positive evidence on gale's exact path that #519 could not provide.

Gate

Behavior-frozen — new fixture + harness only, no codegen change. Frozen anchors
(control_step 0x00210A55, flight_algo 0x07FDF307) and the const_cse flag-off golden
are untouched (frozen gate 3/3, golden 2/2). Full differential passes (exit 0):

--- direct-path (--relocatable) gate, const_cse_direct.wat (#242) ---
OK      r1: 44 -> 40 B (-4) — CSE fired (direct path)
OK      r2: 38 -> 34 B (-4) — CSE fired (direct path)
--      rneg: 46 B (inert)
direct-path gate non-vacuous: CSE fired on 2 function(s), all results match wasmtime, none grew

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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