A parameter cannot say it is required: omitting it renders a default nobody chose

[fcapolini]

Split out of #31, which its own narrowing comment calls "the one with no design space to speak of and the clearest failure today". #31 keeps the value-level half.

The gap

A <:define> can give a parameter a default. It cannot say there is no sensible default and the usage site must supply one.

<:define tag="ui-btn:a"
  ::variant=${'primary'}
  ::label=${''}          // required in practice, no sensible default
  class=${`btn btn-${variant}`}
>${label}<:slot /></:define>

<ui-btn id="m" />

Renders <a class="btn btn-primary" id="m"> with an empty label, and nothing anywhere says why. Verified against main.

This is the last of #31's original five rows that is both silent and name-level. typo-n was closed by ::; the two that remain beside this one (::variant="primry", ::variant=${42}) are about the value, and stay in #31.

Proposed spelling: a declaration with no default is required

No new syntax. ::label in a <:define> declares a required parameter; ::label=${…} declares an optional one with that default.

What it costs, measured. Across the 31 files in kits/ that contain a <:define>, there are 251 :: parameter declarations and every one carries a default. Not one bare declaration exists, so nothing in either kit changes meaning.

What the shape means today, and why it is worth taking. A bare declaration currently means default true:

<:define tag="ui-btn:a" ::label class="btn">${label}|</:define>
<ui-btn />   <!-- renders: true| -->

That is the wrong default for the only thing bare declarations would plausibly be used for. Boolean flags default false — bootstrap-kit/parts/badge.htm declares ::pill=${false}, and every flag in the kit follows. So the shape being repurposed is unused and currently backwards.

Note the bare form at a usage site is unaffected and keeps meaning "pass true" — <bs-link ::button ::outline>. Different position, no ambiguity: one is inside a <:define> tag, the other on an instance.

Why not ::label! or ::label!=

Both were considered and neither pays.

• ! is already spoken for. skipName()'s doc comment in packages/core/src/html/parser.ts lists +/! as the composite-attribute operators (class+=, class!=), and class!=/style!= is still an open TODO. A second, unrelated meaning for ! in attribute position is the overloading the no-magic policy usually refuses.
• != has no right-hand side to take. Required means no default, so ::label!=${x} is self-contradictory. The only coherent form is ::label!, which is a bare name with a sigil — i.e. the no-default rule above, plus a character that has to be explained.

What the error should say

At the usage site, naming what is missing and where it is declared — the shape the existing parameter diagnostics already use:

<ui-btn> requires "::label", declared in ui-btn.htm:4

Compile-time only. Nothing to check at runtime: every usage site is known.

Out of scope

Value and type constraints stay in #31. This issue is only about presence.