Let the front-end declare which blocks board_ui paints

[nbenn]

Summary

• The new initial_block_ids() generic is where a front-end declares which blocks board_ui() paints up front. The board method answers with every block, so core's own UI is unchanged; blockr.dock's answer moves out of its board_ui() method and into a one-line method of its own.
• The board server seeds its build ledger from that same declaration, so the set is stated once rather than restated server-side. That ledger is the FALSE middle state of visible which Make visible a logical channel — is_visible should be isTRUE, not !is.na #306 introduced and nothing in core read until now: NA never built, FALSE built off screen, TRUE painted.
• Reading the ledger back is the new built_block_ids(). A front-end that builds block UI lazily stops reconstructing it — blockr.dock inferred it from !is.na(visible) behind a private helper, and took a real bug (a blanked view on first visit) getting there off required first.
• Core also marks a block built once insert_block_ui() has put it in the page, and marking is one-way, so nothing demotes a painted block back to unbuilt.

Breaking for a front-end that paints a subset of blocks without declaring it: the default method reports every block, so core enters blocks in the ledger whose UI was never built. Confirmed on blockr.dock, where a first visit to an off-screen view then short-circuits its card build and leaves the panels blank. The adoption is BristolMyersSquibb/blockr.dock#442, pinned below.

Not in scope: a core entry point for the deferred build itself. With dock#438 no front-end defers card builds, so such a verb would have no caller; what lands is the vocabulary that makes deferral expressible — declare a subset, insert the rest through insert_block_ui(), report built — with placement left to the front-end that owns its page structure.

BristolMyersSquibb/blockr.dock#442
BristolMyersSquibb/blockr.dag@core349-pin
BristolMyersSquibb/blockr.assistant@core349-pin

Fixes #349

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
R/block-server.R	96.64% <ø> (ø)
R/board-server.R	96.84% <100.00%> (+0.02%)	⬆️
R/board-ui.R	91.00% <100.00%> (+0.27%)	⬆️

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[nbenn]

Closing as superseded by https://github.com/orgs/BristolMyersSquibb/discussions/489. The build ledger in visible leaves core, since a block's UI now exists whenever its server does (#317). A front-end's opening screen becomes its opening claim, declared when its callback is registered under #337's rework, and whether board_ui() paints it into the page is left to #317.