board: CubeCL/LLVM boundary decision, CubeCL audit regrade, pinned _sym merge law

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This PR only adds board records; there are no code changes.

New entry: 2026-09-23-cubecl-llvm-boundary-and-audit-regrade

The decision (operator):

• The T0 fold-before-dispatch law, the masking substrate (bit planes, ternlog, the keyed-reduction family, ndarray::simd) and R2IL are ours. They are never refactored toward CubeCL or LLVM.
• CubeCL is lab-only: a place to take shader and scheduling ideas from.
• The scheduler may borrow ideas (rayon vs CubeCL, bare-metal folding, Mississippi Queen board economics, hexagon), rebuilt as our own scheduler sitting above the fold.
• LLVM is allowed only as a parallel compiler arm, fed the same folded program. JIT codegen and folding into a precompiled polyfill are different philosophies, and neither reshapes the other.

Regrade of the earlier CubeCL scheduler audit (it was never on the board):

Section	Outcome
§1–2 lance-graph analysis	Kept
§3 CubeCL scheduler mechanics	Kept as lab notes
§4 overflow and deadlock hazards	Downgraded to anti-patterns
§5 tier picture	Rewritten. It had drawn CubeCL IR → LLVM as a second path beside T0, with CubeCL's kernel types as the seam. The corrected picture has one T0 path, our own dispatch unit (program + requirements + extent + affinity), and the LLVM/CubeCL arm as a comparison-only branch.
§6–7 invariants and falsifiers	Retargeted at our future scheduler
Proposed CubeCL PR	Dropped

Laws for the future scheduler: exactly-once, barrier affinity, pool isolation, blocked-doesn't-block, and extent-merge-by-fold-algebra. The scheduler itself is still gated on OQ-5.

Also recorded:

• The operator's framing of the HAVING residue: group slot + reserved empty code + exact fold kernel + presence normalization at exit.
• The twin path (_sym SUM vs full-range SUM + COUNT) is a semantic witness, not debt.

Open: cognitive-shader-driver and delta dispatch. The driver recomputes on every cycle. stockfish-rs's incremental accumulator is the reference for delta-driven dispatch. Stockfish's own search doesn't use that accumulator yet, and the entry says so.

TD-SYM-SUM-MERGE-IS-NOT-ADDITION-1: pinned merge law

With ⊥ = SYM_EMPTY_I64, merging partial results follows:

• ⊥ ⊕ x = x ⊕ ⊥ = x, and ⊥ ⊕ ⊥ = ⊥;
• otherwise wrapping_add.

Two consequences are written down with it:

• The row bound covers the combined total. GROUP_SUM_SYM_MAX_ROWS applies to the total rows across all merged partials, not to each partial.
• probe: pin overflow law for SUM-over-add scheduler rewrite #1263's row-first wrapping law is preserved. Wrapping addition is associative and commutative, so merging partials gives the same result as folding all the rows in order.

This is recorded as a law, not code: nothing is built until partial aggregation actually enters the execution path.

Checks

• append_only_gate passes: no protected file shrank (8 checked).
• citation_decay: no new decay.
• Entries index regenerated, and the supersession index regenerated last.
• Every cited file and symbol was re-read before committing. Citations name symbols rather than line numbers, because quack: HAVING over the K sinks, with a NULL-preserving grouped SUM #1266 moved the quack lines.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GXUahz73MZxtxWcfpHp9dG

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Generated by Claude Code

Summary by CodeRabbit

• Documentation
  • Updated project planning records and tracking counts.
  • Clarified guidance for combining partial results and documented compiler and scheduler boundaries, including criteria for future scheduler work.
  • Added a decision entry to the index. No end-user functionality changes are noted.

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📝 Walkthrough

Walkthrough

The PR updates board coverage counts, adds a technical-debt note for SumSymI32 partial merging, and records revised compiler-boundary, scheduler, fold, and execution-reference decisions. It also adds the new decision entry to the board index.

Changes

Execution and aggregation decisions

Layer / File(s)	Summary
Fold and aggregation semantics .claude/board/TECH_DEBT.md, .claude/board/entries/2026-09-23-cubecl-llvm-boundary-and-audit-regrade.md	The technical-debt record specifies the SumSymI32 partial-merge law and its sentinel limitation. The decision record describes HAVING residue and SUM paths as semantic witnesses.
Compiler boundary and audit model .claude/board/entries/2026-09-23-cubecl-llvm-boundary-and-audit-regrade.md	The decision preserves the fold law, masking substrate, and R2IL; limits CubeCL to lab comparison; and revises the audit and tier model.
Scheduler guidance and execution references .claude/board/entries/2026-09-23-cubecl-llvm-boundary-and-audit-regrade.md	The entry records scheduler constraints and splitting guidance. It also notes synchronous driver execution and that the NNUE reference is not wired into search.

Board coverage and entry indexes

Layer / File(s)	Summary
Coverage counts and entry listing .claude/board/SUPERSESSION-INDEX.md, .claude/board/entries/README.md	Three board-coverage numerators increase by one. The entries index adds the new decision record and updates its entry count.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Suggested reviewers: claude

Merge Risk: 🔵 Low · up to b65af

The decision record overstates coverage of an extreme-value case. Clarify that claim; the documentation-only change otherwise appears mergeable.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the three main documentation changes: the CubeCL/LLVM boundary decision, the CubeCL audit regrade, and the pinned _sym merge law. It is specific and relevant to the cha…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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_{A rabbit read the fold-law page,
Then hopped through rows from stage to stage.
Three counts grew, one entry joined,
The scheduler’s rules were clearly coined.
I thumped my paws and closed the sage.}

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Inline comments:
In @.claude/board/entries/2026-09-23-cubecl-llvm-boundary-and-audit-regrade.md:
- Around line 82-83: Update the wording around the COUNT semantic witness to say
that the test checks the presence invariant that would expose a genuine i64::MIN
sum being mistaken for emptiness; do not claim that the test exercises a present
i64::MIN sum.

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• .claude/board/SUPERSESSION-INDEX.md
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