Feature: declare the beta1 supported matrix #426

Problem

The beta1 exit criteria in ROADMAP.md require declaring the supported matrix before verification: exact platforms, runtime versions and capabilities, with every other combination marked experimental or excluded. No matrix is declared yet, so the other beta1 gates have no defined scope.

Desired outcome

A declared beta1 matrix, recorded in docs/verification.md, that the remaining gates verify against.

Scope and non-goals

Maintainer decision (2026-11-12):

• Pi is experimental because Pi refused unverified external guidance on macOS in the alpha20 deployment, and Bug: Pi internal retry recovery is discarded after the first failed attempt #396 (Pi retry recovery) is open. If both are resolved and verified before beta1, Pi moves to supported without changing the rest of the matrix.
• Exact runtime versions are pinned when verification runs.
• Non-goal: full eleven-family by three-runtime certification.

Acceptance criteria

• docs/verification.md declares this matrix before any beta1 verification result is recorded.
• Each supported combination (2 platforms x 2 runtimes x 12 capabilities, 48 in total) records structural checks, isolated install and readback, and one real capability invocation, with command, exit status and bounded redacted output.
• Experimental and excluded combinations are listed explicitly, with the reason.

Evidence

ROADMAP.md, "Beta1 exit criteria"; docs/verification.md, "Alpha20 delivered evidence" (published archives for macOS ARM64 and Linux AMD64; Pi guidance refusal on macOS).