[K3] Reduce 433-1b cell 11 to an exact compact tower

[scottdhughes]

Summary

This stacked K3 packet turns the public 433-1b cell-11 pivot pilot into an exact guarded compact presentation over F_2130706433.

The public pilot records quotient_exact=false at every pivot, so its original nine-generator projection cannot support a closure argument. This packet keeps that negative fact explicit and supplies a replacement at source sign epsilon=(-1,-1), pivot 1:

• three reduced three-equation towers, each reproducing all eight common-locus equations exactly;
• an exact classification of both leading-coefficient boundary ideals on the selected chart, with no deployed-field point;
• a primitive eight-coordinate coefficient kernel whose ten Vieta row products all reduce to zero;
• a canonical source-pinned certificate and 16 mutation tests.

The selected c_row=5 tower has basis size 43. Its two boundary quartics each factor as one linear factor times an irreducible cubic; at the unique deployed r lift, the induced quadratic in b has a nonsquare discriminant. Sage and Wolfram independently reproduced these finite-field checks.

Status and scope

This is deliberately EXPERIMENTAL_REVIEW_REQUIRED / YELLOW. It is PR-worthy structural progress toward the sole remaining exported role orbit [11], but it does not compute the complete signed-pair resultant, establish sign or outside-role transport, perform labeled add-back, project to distinct affine slopes, move a v4 ledger atom, or close K3.

The packet is stacked on #1153, which is stacked on #1152. It does not duplicate another open cell-11 PR.

The falsified FLOOR-v2 random-word first-moment route is not used. No bound for the exact sparse-layer maximum S_sparse is claimed.

Validation

python3 -m py_compile <both new scripts>                     PASS
full local SymPy/Singular replay                            PASS
certificate assembly and verification                      PASS
16/16 hostile mutations rejected                           PASS
second deterministic raw replay (identical SHA-256)        PASS
Sage quartic factorization and nonsquare replay             PASS
git diff --check                                            PASS

Fresh independent review is still required before using this packet in a cell-11 orbit closure.