fix(design-sync): complete the Preact value-import surface so the registry can bundle

[rickylabs]

Summary

deno task design:sync could not bundle the current fresh-ui registry at all. This fixes the converter and makes the failure mode loud.

The bug

The registry gained an mcp-ui-widget island that imports h from preact as a value. The converter rewrites registry preact imports onto a synthetic __ds/preact-compat.ts shim — and that shim had no h export, so the emitted package could not bundle:

conversion errors:
  ! mcp-ui-widget: unmapped preact value import "h" in islands/McpUiWidget.tsx
error: No matching export in "__ds/preact-compat.ts" for import "h"

What changed

1. The shim, not the symptom. The compat shim now covers the Preact value surface the registry actually imports — not a special case for h.
2. Loud failure. An unmapped value import now fails inside convertUnits naming unit, file, and symbol — before deno bundle is reached. A bundler error is a worse diagnostic than the converter's own.
3. Regression tests for both the positive path (a value import converts and bundles) and the negative path (an unmappable symbol fails with a useful message).

A defect caught in review, worth recording

The slice also improved the idempotence tree-hash separator — a genuine improvement — but wrote literal NUL bytes into mod.ts instead of the \0 escape. Git then classified a TypeScript file as binary (Bin 6321 -> 6236).

Every gate was green while that landed: deno check, lint, fmt, the unit tests, parity, all six trap checks, and the idempotence gate — because the hash is self-consistent, so idempotence structurally cannot see it, and the TS parser tolerates NUL inside a template literal. Only reading the diff caught it. Fixed to `${p}\0${c}\0` — identical runtime semantics (the idempotence hash is unchanged, which is the proof), plain-text source, reviewable diff.

Validation

• deno task design:sync check → PASS: parity green, idempotence PASS (f0714aeb10ab), all six trap checks present, 184-file bundle.
• Two pre-existing WARNs, unrelated to this fix: theme-toggle weak props; mcp-ui-widget predicted blank (requires src/theme).
• deno check / lint / fmt clean over tools/design-sync.

Why it matters

This unblocked the beta.10 dashboard work: the NS One design system has since been re-synced with 184 files from today's real registry, so Claude Design generates against current components rather than a stale snapshot.

⚠️ CI note (see #774)

ci.yml only triggers on PRs into main, so check-test and quality do not run on this PR. The green ticks here cover surface-diff + code-quality only. The gates above were run locally; the first honest CI verdict for this wave is the feat/beta10-integration → main PR.

Harness

Codex slice, reviewed by the orchestrator (opposite family) — the diff was read, not the verdict trusted. Run dir: .llm/runs/beta10--orchestrator/, slice design-sync-preact-compat. Not self-certified.

🤖 Generated with Claude Code

https://claude.ai/code/session_01HTiQfrNFCVhjQFqbLKm5xo