fix: radio value clobbering + validation controller path resolution (#13 #12)

[mhenrixon]

Summary

Closes two client-side-validation / binding bugs, two commits on one branch.

Commit	Issue	Type
fix(components): Field#radio keeps its own value, not the model's	#13	🐛 Bug
fix(engine): validation controller identifiers resolve via lazyLoadControllersFrom	#12	🐛 Bug (⚠️ breaking)

#13 — Field#radio clobbered every radio's value

field_attributes carries value: field_value (the model's current value) and
was splatted after the explicit positional radio value, so field_value won.
A new record lost the value entirely (<input type="radio" name="user[role]">
with no value); an edit form gave every radio in the group the same value.

radio now does field_attributes.except(:value).merge(options) — each radio
keeps its own positional value, and an explicit value: option still wins. The
sibling checkbox / toggle / file builders already did .except(:value), so
radio was the only one missing it (no unfixed sibling bug — confirmed by review).

#12 — validation Stimulus controllers never connected

data-controller="forms--validations--length" derives (via stimulus-loading's
controllerFilename) the import path
phlex_forms/controllers/forms/validations/length_controller — but the files
ship at phlex_forms/controllers/validations/length_controller (no forms/
segment). The import 404'd, controllers never connected, client-side validation
silently did nothing.

Fix: drop the forms-- prefix. CONTROLLER_PREFIX is now "validations", so
data-controller="validations--length" resolves to the shipped path. The change
cascades from that one constant to the emitted identifier and the
data-validations--*-value attribute names (data_key reuses the prefix).
Everything coupled was updated to match:

• form.rb derives the validations--form coordinator id from the constant,
  so form-level and field-level identifiers can't drift.
• JS: the invalidate:validations event, the validations-- filter in the
  coordinator, and the data-validations--error / data-validations--counter
  markers.
• Specs, README wiring note, and the docs page.

⚠️ Breaking: hosts that registered forms--validations--* controllers
explicitly must update the identifier to validations--*. (Pre-1.0; the
documented lazyLoadControllersFrom path was broken before, so anyone relying
on lazy-loading was getting no validation at all.)

Test Coverage

• Bug: Field#radio lets field_attributes clobber the positional radio value #13: keeps each radio's own value; checks only the matching radio; explicit
  value: still wins (3 new specs).
• Bug: validation Stimulus controllers can't be resolved by lazyLoadControllersFrom #12: introspector emits validations--presence / validations--length +
  the validations__*_value data keys; components_spec asserts the coordinator,
  submit action, and per-field controllers all use the new prefix.

Verified independently: every emitted identifier resolves to a shipped
validations/*_controller.js; the JS dispatch/listen event names and the
write/read marker strings are internally consistent; a repo-wide grep finds zero
surviving forms--validations / forms__validations references (only the
intentional CHANGELOG migration note).

Full suite: 144 examples, 0 failures · rubocop lib spec clean.

Deviations & judgment calls

• Bug: validation Stimulus controllers can't be resolved by lazyLoadControllersFrom #12 chose option 2 (rename identifier) over option 1 (move files) — your
  call. Option 1 (move controllers under forms/validations/) would have been
  non-breaking, but option 2 keeps the files where they are and shrinks the
  identifier. Justified pre-1.0, and the previously-documented lazy-load path was
  already broken, so there's no working host wiring to break.
• form.rb derives the coordinator from CONTROLLER_PREFIX rather than
  repeating a literal "validations--form" string — so the form-level and
  field-level identifiers share one source of truth and can never drift on a
  future rename.
• Bug: Field#radio lets field_attributes clobber the positional radio value #13 needed no sibling fix: an adversarial review pass confirmed
  checkbox/toggle/file already drop the model value, and input/textarea/hidden
  intentionally keep it. Radio was the sole outlier.