docs(plans): record the maintainer's 2026-10-08 decisions in the v2 plan

[pdlourenco]

Summary

Records the maintainer's 2026-10-08 decisions in docs/plans/2026-10-04-review-v2-remediation-plan.md.

Stacked on #206. This PR is based on #206's branch, because the §0.2 decision table it records against exists only there. Merge #206 first. GitHub then retargets this PR to main, and its diff becomes these three commits.

All seven decision items the must-do set needs are now taken. They are logged in the new §0.3 and marked in the inventory, §0.2, item texts, §9 and §10:

• A1: require the six listed contexts plus the fork-PR token guard; keep the admin bypass.
• A2: option (i), no commit on main; (i-a) the PR's MATLAB job pushes regenerated vectors to the PR branch, (i-b) the maintainer commits them for fork PRs; PR-time ancestry and canonical-engine provenance check. The maintainer's "A2 — agree" is read as covering all three §0.2 A2 rows, and the log says so.
• A3: string sentinels for NaN/Inf on disk.
• A6: if the IO path is engine-sensitive, pin the cost trajectory and iteration count.
• A10: the documented floor stays Octave 11; both Octave CI jobs run in a gnuoctave/octave:11.3.0 container, the version on kynes, instead of the unpinned snap. The suggested docs wording ("GNU Octave 11+, CI: 11.3.0") is marked as not yet confirmed.
• D2: the §14.5/§15.4 additions.
• E1b IO cost fields:
  • The IO per-iteration history is renamed CostHistory / cost_history, accepting the released-API break (0.x, no user base). This reverses the plan's recommendation to keep the name.
  • Cost becomes the §8.5 (1 × 3) [total, data_fidelity, regularization] breakdown at the returned estimate, so Cost(1) == CostHistory(end).

One sequencing choice left open. A6 can store CostHistory and the new Cost only once E1b's IO cost-field change is in place, and the rest of E1b still waits on open rows. The plan recommends carrying that one change in A6's PR, spec commit first.

Spec sections touched

none — no algorithmic change (plan document only). The decisions it records change §14.5/§15.4 (D2) and §8.12.9 (E1b) when their item PRs land, spec first.

Type of change

• Docs / tooling / CI only

Checks

• Major / architectural decision made? Recorded in the plan's §0.3 decision log. The superseding ADRs (A1 vs ADR-0005; A2/A3 vs ADR-0002 and ADR-0005's bot bypass) land with their implementing PRs, per the plan's §0.2 preamble. For A10 and E1b's IO cost fields the plan recommends no ADR.
• Pre-push review: four rounds.
  • Round 1 flagged 4 blockers. The record over-claimed A10 and the CostHistory name as decided before the maintainer confirmed them. The commit was rebuilt from scratch.
  • Round 2 found no blockers and 6 nits, fixed in 175e511.
  • Round 3 found no blockers and 8 nits, fixed in 7cbc910.
  • Round 4, on the Cost decision, found no blockers and 3 nits, plus the A6 sequencing gap. All fixed in ce22ad4; the gap is recorded as a recommendation, not a decision.
• Local CI: docs-only plan edit outside the published site; table cell counts checked on every edited table. CI runs once this PR targets main.

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