doeff-hy: 3-arg bind type annotation (<- x str expr) is not enforced at runtime

[proboscis]

Observed

(<- x str (Pure b"raw-bytes")) binds x = b"raw-bytes" and continues silently — the declared str type is never checked. Minimal repro (against main 678c184):

(require doeff-hy.macros [defk <-])
(import doeff [Pure run])

(defk check-it []
  {:pre [True] :post [(: % bytes)]}
  (<- x str (Pure b"raw-bytes"))   ;; declared str, receives bytes
  x)

(print (run (check-it)))            ;; prints b'raw-bytes' — no error

Why it matters

• The documented contract ("(<- x str (Ask ...)) — yield, bind, assert isinstance") promises a runtime check, and defk's :pre/:post contracts ARE strictly enforced (the macro even rejects object as too broad) — so the silent no-op on the bind annotation is an inconsistent gap in the contract system.
• Real bug it hid today (proboscis-ema ISSUE-TRD-176 Phase 0c): (<- api-key str (GetSecret ...)) silently bound the GSM payload bytes; the type error only surfaced deep inside the lighter SDK on the first real-testnet run (startswith first arg must be bytes...). An enforced bind annotation would have failed loudly at the boundary in L0 tests.

Expected

Either enforce the 3-arg bind annotation with the same isinstance machinery as :pre/:post (preferred), or reject the 3-arg form at macro-expansion time so the annotation cannot masquerade as a check.