Introduce contentRole on blocks

[janglad]

Pen decides whether to show the document placeholder based on whether the document is
empty, which previously worked by counting root blocks. That is too limiting: some blocks
(an email signature) are chrome the host puts in the document, and
should not count toward the document having content. With one present, the placeholder
never painted — and since the same boolean decides where a click below the blocks lands,
the caret went into the chrome too.e

Pen cannot infer this from what schemas already declare: divider carries the same
fieldEditor: "none", selectionRole: "structural", and flowCapability: "flow-structural"
as a signature block, and [empty paragraph, divider] is a document with content in it.

Fixed by introducing authoring.contentRole on the block schema, letting a host say
whether a block type counts toward a filled document. Defaults to "content", so nothing
changes for a host that does not set it. documentState.isEmpty and data-empty are
untouched. Specified as RI8 in spec/rules/dom.md (document placeholder visibility had no
rule before), evidence in ri8DocumentPlaceholder.test.ts, patch changeset.

---

Note

Medium Risk
Touches placeholder eligibility, pointer click-below caret placement, and a new public schema field across DOM and framework bindings; behavior change is opt-in via explicit contentRole: "chrome".

Overview
Adds authoring.contentRole ("content" | "chrome") so hosts can mark furniture (e.g. signatures) that should not count as user-facing document content. getBlockContentRole in @input/pen-core reads it and defaults to "content".

Document empty-state handling shifts from counting root blocks to naming a target block: getDocumentPlaceholderTargetBlockId (@input/pen-dom) replaces computeDocumentPlaceholderVisible. Eligibility ignores chrome, requires exactly one empty inline editable content block, and returns that block’s id (or null). resolveInlinePlaceholderVisibility now takes isDocumentPlaceholderTarget instead of isFirstBlock + isDocumentEmpty.

React and Vue paint emptyPlaceholder on the target block (not blockOrder[0]). Click-below-block gestures activate the target via the same helper. documentState.isEmpty / data-empty are unchanged. RI8 is documented; patch changeset across pen-types, core, dom, react, and vue.

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