fix: keep runtime sheet intact when the CSSOM rejects a rule

[dbanksdesign]

Problem

Two runtime-injection bugs compound to render consumer apps completely unstyled in real browsers (confirmed in Chrome while integrating var-ui's component-breadth work):

1. Invalid @property emission. registerAtPropertyRule writes initial-value: ${value} verbatim. Per CSS Properties & Values Level 1, an @property initial value must be computationally independent — var() / env() references are not allowed, so CSSStyleSheet.insertRule throws SyntaxError. Every c.vars() recipe var or registered property whose default is a token reference (var(--…)) hits this.

2. Destructive text fallback. When insertRule throws, flush() / writeAllRulesToStyleElement appended the rejected rule as a text node to the same <style> element that holds the insertRule-built sheet. Mutating a style element's text makes the browser re-parse it from text content, discarding every rule previously added via insertRule. Net effect: one rejected rule wipes the entire runtime sheet. Observed end state in Chrome: document.getElementById('typestyles').sheet.cssRules.length === 0, page fully unstyled, while the element's text held only the rejected @property rules.

jsdom does not validate @property, which is why unit tests never caught this.

Fix

1. When a registered property's initial value contains var( / env(, degrade to the universal syntax — @property --x { syntax: "*"; inherits: <inherits>; } — which is the only form allowed to omit initial-value. Type checking is lost for those properties, but inherits (the load-bearing behavior) survives, and the default still reaches the cascade through the existing :root / base-style assignments. Computationally independent values keep the typed rule exactly as before.

2. Route all text fallbacks (flush, doc-swap rewrite, layer-order preamble) to a dedicated <style id="typestyles-fallback"> element placed immediately after the main element. The fallback element only ever holds text — never insertRule — so re-parsing it on append cannot lose rules. It mirrors the main element's detach/reconnect lifecycle.

Tests

• registered-property.test.ts (new): typed rule preserved for literals; universal degradation for var() / env(); :root default assignment unaffected.
• sheet.test.ts: patched CSSStyleSheet.prototype.insertRule to reject marked rules — asserts previously inserted CSSOM rules survive, rejected rules land in the fallback element, the main element gains no text nodes, the doc-swap rewrite path stays non-destructive, and the fallback sits right after the main element.

pnpm verify passes (43/43 turbo tasks; 438 typestyles tests). Changeset included (patch).

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