v1.5.1 — checkbox state was unreadable on REDCap 17 (live-found, pid 149)

Bug fix. Anyone using a checkbox in a when/assert condition, or filtering a
checkbox with @UVCHOICES, should take this release. Found on the first live
run of 1.5.0 on REDCap 17.0.6.

• The defect. For each checkbox option, REDCap 17 renders TWO elements: a
  hidden input named __chk__<field>_RC_<code> (its VALUE is the code when
  checked, "" when not — type=hidden, so its .checked is always false) and
  the visible clickable <input type=checkbox> carrying id
  id-__chk__<field>_RC_<code> (shared name __chkn__<field>). The engine read
  .checked off the element it found by name — the hidden mirror — so a
  checked box read as unchecked. Consequence: every [field(code)] checkbox
  reference evaluated false regardless of the real state. A cascade gated on a
  checkbox (e.g. @UVCHOICES={"when":"[pilot(1)]='1'",…}) never activated; a
  checkbox @UVASSERT/@UVREQUIRED/@UVUNIQUE when never fired; a
  checked-but-hidden @UVCHOICES code was never detected as stale.
• The fix. A single QRID_readCheckbox(field, code) now resolves the state
  across renderings: a __chk__…_RC_code that IS a checkbox → its .checked
  (classic REDCap); one that is hidden → value === code (17.x mirror); else
  the visible id-__chk__…_RC_code checkbox → its .checked. readRef routes
  every checkbox reference through it, requestField also binds change/click on
  the visible __chkn__<field> (the hidden mirror fires no events), and the
  @UVCHOICES renderer computes "is this option checked" the same robust way so
  a checked option is never hidden from under the user.
• Why the tests missed it. The DOM stub modeled __chk__…_RC_code as a real
  checkbox with a working .checked — more forgiving than REDCap 17. The stub
  now models the 17.x two-element structure (hidden mirror + visible box in one
  choicevert row); tests/choices_dom_js.cjs gained a checkbox-ref-gated
  cascade and a checked-hidden-stale case (58→67 checks) that FAIL on the old
  code and pass on the fix (verified by reverting). Backward compatibility with
  the classic single-checkbox rendering is retained and still covered.
• Radio, dropdown, the two-level cascade, stale-kept selections, and the
  save-block (off/confirm/hard) were all verified working live on the same run.

Known issues (not fixed here)

• Performance on rule-heavy projects. A project injecting ~69 rules made a
  checkbox click freeze the page for tens of seconds live (each rule installs its
  own document.body MutationObserver; a click that mutates the DOM fans out to
  all of them). Pre-existing and module-wide, not specific to choices mode —
  tracked separately. Candidate fix: inject only rules whose fields are on the
  rendered instrument, and share one observer.
• Checkbox message placement. A @UVCHOICES message on a checkbox field is
  anchored inside the first option row; a show/hide list that hides that first
  code could hide the message with it. Narrow (typical cascades keep the first
  code shown); to be re-anchored above the option rows in a later release.

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Universal Field Validator is a REDCap external module. Rule kinds: @UVALIDATE (check-character / regex), @UVASSERT (cross-field constraints), @UVREQUIRED (conditional required), @UVUNIQUE (no duplicates across records), @UVCHOICES (dynamic choice filtering).

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