docs(spec): record the attention aggregation direction (candidate) (#81)

[Xuepoo]

Priority: P1 | Area: UI, plugins | Labels: docs, P1, area:ui, area:plugins | Milestone: v0.1.0 | RFC: - | Task: CTX-0044

Summary

Records the candidate attention aggregation direction in U-10 of the Lua UI component model: where attention requests converge, why the counting half cannot live with the requester, and what stays open. Single-file change.

Deliverable

extensibility/lua-ui-component-model-candidate.md:

• Attention aggregation direction (candidate) paragraph added after the five-part U-10 list: the counting half (registration, deduplication, ordering, expiry) belongs at a host-side point — no plugin sees the full request set, so a per-requester counter cannot enforce the never-escalate rule against other requesters. The candidate split: participants raise one request primitive; a host-side aggregation point registers/deduplicates/orders/expires under a host-admitted budget; the terminal-side notification surface renders what it admits.
• Narrows, does not close: the recorded open question ("whether attention is a plugin counter or a host service", cross-linked to the UI runtime candidate) keeps its ownership decision open; which owner provides the aggregation point, the budget shape, and expiry semantics stay open.
• U-10 Open paragraph now also lists aggregation ownership among the undecided items.
• Verification item 8 now tests aggregation through the host-side point rather than per-requester counters.

Owner direction applied (2026-09-21)

• Sequence: follows the Wheel-side agent-attention mapping (bitty-ai-docs PR #169); this is the plugins-corpus half.
• Three-layer reading confirmed: participants raise one primitive; counting half is host-side; presentation stays with the terminal-side notification surface.

Scope discipline

• Candidate status only; no implementation claim; no budget number; no capability identifier.
• The cross-repo question is narrowed in direction only; ownership stays open with its corpora.

Gates

• just check — exit 0 (70 markdown files validated).
• act -n --action-offline-mode — both jobs succeeded.

Closes #81