Step 17 of v4.3 playground plan. Pure function relocateAnchorsToBlockBoundaries
(text, anchors) detects atomic markdown blocks (fenced code, tables, deeply
nested lists) and relocates anchor-comment insertion to the line BEFORE block
opening rather than inside the block. Pure markdown-text -> markdown-text
transform (no DOM, no markdown-it dependency).
Companion test tests/integration/annotation-block-boundary.test.mjs extracts
the function via balanced-brace scan and exercises it through Function() —
7 unit tests covering empty anchors, outside-block stays, fenced-code
relocation, table relocation, deeply-nested list relocation, mixed
inside/outside, and shape contract.
Trace: SC6, research/04 Dim 3 (Notion block-level fallback), plan-critic
major #6 (DOM-vs-no-DOM contradiction resolved via pure-function design).
