Feature: add two-way data binding via `::prop` in `comark` core

[miguelrk]

Description

Comark data binding is read-only. :prop="dot.path" resolves against the render context; {{ }} and ::if read the same context. There is no write channel.

Add ::prop="dot.path" in comark core so any component or native form element can write back to a small ComarkModel protocol. Native HTML form elements become the input surface — no first-party Inputs library.

Volume: :input{type="range" min="0" max="100" ::value="data.volume"}

The volume is **{{ data.volume }}%**.

::if{:value="data.volume" :gte="90"}
That's loud.
::

::prop is attribute-position only. It does not collide with block ::name, inline :name, one-way :prop, or {{ }}. The props parser currently drops ::value, so the namespace is unclaimed.

This belongs in core, not @comark/binding. Two-way binding extends the attribute-resolution contract in resolveAttributes() / NodeRenderData, consumed by all renderers.

flowchart LR
  MD["Markdown<br/><b>::value=#quot;data.volume#quot;</b>"]
  Core["comark core<br/>parse · resolveAttributes"]
  Model["ComarkModel<br/>get / set / subscribe"]
  UI["Renderers<br/>input · Binding · If"]

  MD -->|parse| Core
  Core -->|"X + onUpdate:X"| UI
  UI -->|set| Model
  Model -->|subscribe| UI

  classDef src fill:#ecfdf5,stroke:#059669,color:#065f46
  classDef core fill:#eff6ff,stroke:#2563eb,color:#1e3a8a
  classDef model fill:#fff7ed,stroke:#ea580c,color:#9a3412
  classDef ui fill:#f0f9ff,stroke:#0284c7,color:#0c4a6e

  class MD src
  class Core core
  class Model model
  class UI ui

  linkStyle 0,1 stroke:#2563eb,stroke-width:2px
  linkStyle 2 stroke:#ea580c,stroke-width:2px
  linkStyle 3 stroke:#0284c7,stroke-width:2px

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Motivation

Interactive Markdown today needs either host-owned wrappers per control or an Inputs-style catalog. Both bypass the render context and do not work for plain HTML. A write at the attribute layer updates every reader of that path ({{ }}, ::if, other :prop bindings) without a component catalog.

Proposed solution

Syntax — Widen the props-key regex to accept ::. Reject a :: key with no value.

:input{type="text" ::value="data.name"}
:input{type="checkbox" ::checked="data.active"}
::dialog{::open="data.showHelp"}

ComarkModel (comark/model) — Dependency-free protocol: get / set / subscribe (optional batch). Ship createModelStore(). Hosts can pass their own implementation (Vue reactive(), Zustand, …). Default store is per render root.

Attribute resolution — For ::X="path" with a model: emit X (current value) + an update callback (onUpdate:X / onUpdateX). Filtered expressions, non-writable paths, and missing model degrade to one-way (warn in prod / throw in dev where appropriate). Reuse existing URL/HTML-sink guards for ::href / ::src / innerHTML. set() rejects prototype-pollution segments and paths outside writable (default: ['data']).

Native form elements — Shared helper wires input / select / textarea (value / checked / files) with type coercion. Custom components receive X + update callback and write themselves.

Renderers — React / Vue / Svelte / Angular bridge via subscriptions. HTML SSR emits inert data-comark-model* attrs; optional @comark/html/runtime for progressive enhancement. ANSI resolves ::X as :X (one dev warning). Markdown gains model / onModelChange; default creates a store from data when no model is passed.

::form — Optional aggregate that collects child binding paths into one object (same layering as Binding / If).

Out of scope

• First-party Inputs library (use native :input{type=… ::value=…})
• Table-as-input / structural widgets (component-library concern; same split as #427)
• Attribute modifiers (.number / .lazy / .trim) — leave . in keys unclaimed
• Shipping a reactivity engine in core (alien-signals etc. stay optional / separate)

Constraints

• #286 — ::prop is inline-attribute-only until YAML block props resolve bindings
• #296 — Angular adapter after signal-input modernisation
• #427 / PR #429 — filtered expressions are not writable
• PR #415 — inside ::for, settle index-aware paths (item.done → data.items[i].done) jointly

Additional context

• Binding docs — one-way contract this extends
• Observable Framework Inputs — catalog this makes unnecessary for primitive controls