docs(rev15): Req 6.13 states the deployed three-member/q=2 beta profile

[cryptskii]

What

Owner-ordered spec amendment (step 1 of the post-e2e cleanup sequence): Rev 15's normative text still described the retired five-member/q=4 draft profile; the deployed fleet is the three-member Alibaba set fixed at q=2 (owner decision 2026-08-23, dsm/src/dlv/beta_storage_profile.rs, hardware-validated by the two-device e2e).

Four spec sites amended in .github/instructions/sofispecs.instructions.md:

1. Requirement 6.13 — now "Three-member beta profile", q=2, one member down ⇒ both survivors required. Durability rationale recomputed honestly: |Q1 ∩ Q2| ≥ 2+2−3 = 1, and with one member unavailable every later two-member quorum consists of the two survivors, so any previously chosen quorum intersects it in at least one member. Stated as the minimal margin, not a bonus margin.
2. §18.2 (Theorem 18.1 discussion) — 2/3 is exactly the strict majority the uniqueness theorem assumes; the earlier 4/5 draft's extra durability margin is recorded as retired with the fleet, with the theorem's assumptions unchanged.
3. Conformance table — quorum-fixed/ and quorum-margin/ rows rewritten for two-of-three.
4. beta_storage_profile.rs module doc — no longer claims "the spec amendment is owed"; records that spec and code now state the same profile. No behavioral change; cargo fmt --check + clippy --all-targets green on the crate.

Verification

grep -ni 'five-member|4/5|3/5|q = 4' .github/instructions/sofispecs.instructions.md — the single surviving mention is §18.2's deliberate historical note that the 4/5 draft was retired.