fix: nest the decision/finding fallback inside the carrier node

[EiffL]

Fixes #9.

For placed decisions and findings the plugin emitted the neutral fallback (the details/summary dropdown; the notes/scope paragraphs and evidence blocks) as siblings of the heading carrier. A rich theme overriding the carrier replaced only the heading, so the fallback still rendered and readers saw every decision and finding twice (see LightconeResearch/astra-theme#2, item 3).

Change

• Decisions and findings now render as one div carrier bearing the <kind>-<id> identifier, with the entire neutral fallback nested as its children — the same pattern the output and prior-insight carriers already follow. A theme replacing the carrier replaces the fallback wholesale; nothing is left behind to double-render.
• The div node is the one the stock :::{div} directive emits, so neutral themes render the children exactly as before, and the identifier still anchors cross-references.
• data.tags moves from the heading to the carrier (the joinable node).
• New carrierDiv() helper in ast-helpers.ts; carrier tests updated to pin the nesting so the sibling shape can't regress.

No changes needed in index.ts: class stamping (tagComponent), :label:/:class: overrides, and registry rendering all locate the carrier by identifier and pick up the wrapper automatically.

Theme follow-up (separate repo, deferred)

astra-theme registers heading[class*="astra-decision"] / heading[class*="astra-finding"]; those selectors need to move to a div bucket once the theme builds against this version. The components need no logic changes — they join by node.identifier and degrade via node.children (the degrade path improves: children are now the full fallback rather than the bare title).

Testing

• npx vitest run — 94/94 pass, including the updated carrier-contract tests.
• npx tsc --noEmit — clean.

🤖 Generated with Claude Code

[EiffL]

Verified end-to-end against astra-theme + the DESI DR1 BAO prototype (local dist/mystra.mjs build of this branch):

• Decisions: rich panel renders from the new div carrier; zero stock fallback dropdowns left in the DOM (previously the theme hid them with a CSS stopgap — now they're gone at the AST level).
• Findings: the double-render is gone — notes/scope text appears exactly once (this one had no theme-side workaround, so it's the big win).
• Anchors: decision-<id> / finding-<id> land on the carrier div and cross-page #decision-… links still resolve.
• vitest 94/94 + tsc clean on the branch.

Theme follow-up is done in astra-theme (uncommitted working tree): renderers registered under a div bucket with the old heading selectors kept for ≤ v0.0.5 back-compat, the decision panel's no-rationale branch no longer renders node.children (that would now nest the entire fallback inside the rich panel), and the CSS stopgap comment now marks it back-compat-only. LGTM.