[M2-DEC-02] Confirm minimal UI widget set (D-UI)

[offdev]

Step

M2-DEC-02 (roadmap M2-rendering-2.5d.md, decision register D-UI, PRD §18 item 5).

What this PR does

Docs only, no code:

• ADR 0006 (docs/decisions/0006-ui-scope.md) — confirms the M2 retained UI widget set as the seven FR-2.8 widgets in canonical order: panel, button, text, image, list, slider, input (M2-UI-01's Verify clause "widget list in ADR == widget list implemented in M2-UI-01" binds exactly this list).
• Flags the PRD §18 item 5 question text's omission of slider as a typo: the normative FR-2.8 list and the register's D-UI default-if-unresolved both carry the seven-widget list.
• Names the deferred widgets per the step scope — checkbox/toggle, progress bar, tab, dropdown, tooltip, multi-line input, separator — each with its deferral rationale and expected landing; the set only grows additively (never breaks the M2-UI-01 surface).
• ADR indexed in docs/decisions/README.md and docs/README.md; M2-DEC-02 box checked; decision register D-UI row closed; Progress Board M2 2/33 (total 49/194); change log line added.

Why this decision

• M2's acceptance chain requires UI (M2-SCENE-01's reference scene panel, M2-UI-02's golden scene, M2-TEXT-02's TextItem "in the UI pass"), so "defer full UI to M4" (the §18 alternative) would orphan M2 steps; M5-ED-01 composes the editor shell (menus/docks) from this set.
• The seven widgets map one-to-one onto M2-UI-01's event list (pointer enter/leave/press/release, button pressed, slider changed, input text); the ADR's per-widget table matches it exactly.
• No performance claim (scope confirmation, no CORE-001 measurement); cost bounded by construction — batched quads + text in the screen-space UI pass, one texture per UI atlas, counted in M2-PERF-01's ≤ 30 draw-call budget.

Verification

• Verify clause: ADR exists ✓; list equality with M2-UI-01 holds by construction now (M2-UI-01 not yet landed — the clause binds its implementation).
• No code changed: no build/test/lint impact (the standard CI suite is unaffected; no new symbols, laige-api.json unchanged).

AGENTS.md §17 checklist (applicable items)

• Scope: docs only, no unrelated changes; no code, no new dependencies.
• Docs updated in the same change (CORE-006/DOC-007); ADR follows the DOC-005 structure.
• No ENGINE-RULE-EXCEPTION added.
• GitHub API via the project-local .env token; the token appears in no commit, log, or output (GHT-001/003).