feat: RNE proof via lower/upper#93
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Reframe the rounding proof around RNE and assemble it in terms of lower/upper. docs/lower-pen-and-paper.typ: retarget to RNE (toExtRat_round_Rel_smtLibRound_of_RNE); Part I proves the two candidates (blastLower/blastUpper), Part II assembles RNE via a selection engine stated purely as lower/upper distances; demote the truncation identity to the one-ulp gap. Delete PLAN_RNE.md (folded into the typst doc). Round.lean: - Selection engine (lower/upper distances): blastIsLowerHalf/TieBreak engine lemmas, spec-side distance lemmas, and the _of_nonneg / sign-general _of_normal selector bridges (glue proven by grind, leaves are sorry). - Found that the predicates are FALSE at an exactly-representable r (circuit and SMT spec disagree there); engine/bridge lemmas now carry the required 'lower r < r' hypothesis, documented inline. - Restructure toExtRat_round_Rel_smtLibRound_of_RNE: peel off the exactly-representable case, and use the corrected _of_normal bridges in the non-representable case (corrected bridges now on the critical path). - Delete the four false unconditional bridges (no external users). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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This PR adds the RNE proof via lower and upper theorems