feat: nested container support

[teezeit]

What this fixes

The previous parser (remark-containers.ts) used a flat linear scan: when it found a :::type opener, it iterated forward collecting sibling nodes until it hit a closing :::. Any :::type encountered during that scan was either treated as plain text content or, in some paths, re-injected into the tree in a way that could produce incorrect output.

This meant nested containers silently didn't work — :::card inside :::modal would appear as a sibling of the modal, not a child.

What changed

Rewrote remark-containers.ts using a recursive collectContainer() approach (ported from the v2-proposal reference implementation):

• When collecting a container's children, any :::type opener found triggers a recursive call to collectContainer() — producing a properly nested AST node.
• Three cases handled cleanly: (1) complete container in a single plain-text paragraph, (2) complete container with inline elements, (3) multi-block container with recursive nesting.
• Implicit same-paragraph closing — handles [Save]* [Cancel]\n::: where the closing ::: shares a paragraph with the last content line (remark folds them when there's no blank line).
• Opener-content extraction — :::alert Warning: this action is irreversible adds the inline text as the container's first child, with the type parsed as alert only.

Bugs fixed

Symptom	Root cause	Fix
:::card inside :::modal rendered as a sibling, not nested	Flat scan had no recursion; nested opener was re-injected at the wrong level	collectContainer() recurses when it encounters a nested opener
[Save]\n::: (no blank line before closer) — closer was missed, container ate subsequent nodes	Implicit closer detection was inconsistent across code paths	Unified implicit-closer handling in Case 3
:::alert Some message parsed containerType as "alert Some message"	Regex ([^{]+?) captured all non-brace chars including spaces	New regex (\S+) stops at whitespace; remainder becomes inline content

Tests added

• should nest a container inside another container — asserts :::card is a child container of :::modal, not a sibling
• should extract inline content from the opener line — asserts ::: alert Warning text produces correct containerType + first child

Docs

Added nesting and opener-content examples to the Block Containers section of docs/guide/syntax.md.

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