Extend drag-and-drop to nested blocks via the content model

[namedgraph]

Summary

Every %block element now carries its own drag handle wherever the XHTML content model places blocks — a region, a cm:flow container (li/dd/td/th/div/figure/figcaption) or a blockquote — so nested blocks (including an RDFa object div in a list item) can be dragged out of, into and between containers.

• One predicate (local:draggable-block) routes every chrome/drag decision: a real %block — never a run wrapper, hr or ephemera. Chrome injection rides the existing local:init-block recursion (a one-gate change); local:ensure-chrome in after-mutation converges handles after every gesture, so mutation sites never thread their own injection.
• Content-model-validated drop resolver: the target is the innermost draggable block under the pointer whose container accepts the dragged kind (local:deepest-block-at + local:legal-drop-level climbing to the nearest legal level — the region accepts any block, so the climb terminates and an illegal drop is clamped, never created). Gutter/gap drops stay top-level, so the left gutter lifts a nested block all the way out.
• Cells take "into" drops: nothing can legally drop before/after a td/th, so a cell's pixels away from any block it holds mean into it — the text host becomes a container per the insert-block-at-caret flow doctrine, marked by an inset drop-into box. A list item deliberately stays a clamp to around-the-list (its pixels compete with reordering).
• Origin repair at commit: prune-husks (an emptied quote goes — B7 doctrine) then collapse-container (a flow container that lost its last block reverts to a text host).
• Undo fidelity: transient drag state is stripped before the snapshot; local:tidy-class kills the latent class="" residue classList.remove left behind (previously poisoned undo baselines after any completed drag); restore-snapshot strips every handle and re-converges, generalizing the table-fostering fix to nested tables.
• Stacked handles per nesting level, each at its block's own gutter indent; the editing view indents blockquote content (plus a Markdown-style quote bar) so quote-child handles don't coincide with the quote's.
• Demo copy, help modal, README and CLAUDE.md updated to the new doctrine.

Test plan

• make test — 49 headless PASS, incl. new canonical (nested-chrome strip at any depth) and lint (nested object-div shapes clean) fixtures
• make test-browser — all 15 suites green, incl. new dragnest.mjs (real-mouse gestures against tests/fixture-dragnest.html: nested handles at init, lift-out of a list item — incl. the RDFa object-div case with triples intact — drop-into a container and INTO a text-only cell, before/after precision on a cell's blocks, legality clamp, region clamp, origin collapse, split convergence, canonical cleanliness; every scenario ends in invariants + single-undo byte-exact restore)
• Invariant I3 updated ("chrome top-level only" → "chrome only on draggable blocks") across the suites' invariant copies; old-contract assertions (noChromeOnP, noChromeInCell, …) deliberately flipped
• Manual: demo page — quote's inner paragraph drags out of the quote; blocks drop into the "Everyday sentences" table's cells

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