fix: render metric embeds on a div carrier with a readable sentence fallback

[EiffL]

Closes the metric leftover from #10 (also tracked in LightconeResearch/astra-theme#2's ecosystem notes).

Problem

The resolved store already materializes metrics (metric: readMetric(absPath) since v0.0.6), but the embed rendering didn't hold up its end of the contract:

• A produced metric (:::{astra} outputs.<id>) rendered as a collapsible key/value table on a details carrier — a poor neutral fallback, and a node type rich themes can't sensibly key on, so astra-theme's big-stat rendering (MetricStat) could never trigger.
• An unproduced output embed (any type) emitted the "Pending Output" admonition with no identifier at all — tagComponent's carrierOf fell back to nodes[0] for the class, but cross-references and store joins had nothing to land on, even though the store entry (label, description, full provenance) exists before the artifact does.

(Side note from testing: the DESI prototype's analyses/reconstruction/results is a broken symlink to NERSC scratch, which is why metrics appeared unexercisable there — worth knowing when testing locally.)

Change

• A produced metric renders as one div carrier (the fix: nest the decision/finding fallback inside the carrier node #11 contract) bearing output-<id>, with a readable sentence nested as the neutral fallback: <label>: <value> ± <uncertainty> <unit>, built via readMetric. A JSON that isn't metric-shaped keeps the existing tabular fallback, wrapped in the same carrier.
• An unproduced output embed wraps its Pending admonition in the identifier-bearing carrier div: anchors resolve, and rich themes can join and decorate (astra-theme now shows the provenance drawer on pending outputs).
• data / report types keep their current inline rendering.

Testing

• Carrier tests updated/added: produced metric pins div + sentence + no details; new unproduced-embed test pins the identifier + nested admonition. vitest 101/101, tsc clean.
• Verified end-to-end with astra-theme against a minimal ASTRA project on this branch: the produced metric renders as the big stat (6.42 ±0.31 Mpc/h + label + provenance drawer) — the first time that component has rendered from real plugin output — and the pending embed keeps its admonition and gains the drawer. Theme-side support (a div[class*="astra-output"] renderer bucket, inert on older plugins) is already on astra-theme main (c56c74a).

🤖 Generated with Claude Code

https://claude.ai/code/session_013EE6NiwH2UhVAUT6cdAQJ4

[EiffL]

LGTM