docs: reconcile numeric-tower grounding status (landed→grounded, post-#5428 + #5425)

[gunbai-bot]

Summary

Deferred doc reconciliation from #5428 (smart-crane) which explicitly noted: "The shared-doc status reconciliation (numeric row landed→grounded; DESIGN §5 open-thread b) is deferred to the lane manager post-#5425." #5425 has now merged.

Two targeted edits, no logic changes:

• DESIGN.md §5 open-thread (b): numeric tower marked GROUNDED (Numeric-tower grounding: ground Nat construction-side so native form == modeled form (§0) #5428, 2026-06-21); records that eval_binop CrossRepresentationEquality guard is dead-in-corpus for numerics but deliberately kept as fail-closed backstop until Value::Null split (guard removal bundled with that work, fenced out of this window). Remaining item: Value::Null split (~131 sites, its own runway).

• docs/plans/model-realization-fork.md §3.1: numeric row updated from "Finish … Start here." (pending) to GROUNDED: exact realization (Zero→Int(0), Succ{prev:Int(k)}→Int(k+1)), discriminating witness noted, guard-kept rationale recorded. §3.2 (Value::Null split) unchanged.

Test plan

• Doc-only PR; no .dag or .rs changes — CI floor runs compile-clean + layering-imports, no functional gates to break.
• Verify §3.1 in model-realization-fork.md matches Numeric-tower grounding: ground Nat construction-side so native form == modeled form (§0) #5428's behaviour (Zero/Succ realizations, guard kept, witness confirmed).
• Verify DESIGN.md open-thread (b) is now a consequence-chain (grounded fact → guard dead-in-corpus → remaining root = Null split) with no orphan claim.

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