Gate canonical App Builder showcase regeneration

[kattsushi]

Part of #131. Follows #146/PR #147 and #148/PR #149 as Unit 7 of the App Builder feature-branch chain.

Problem

PR #130 must not be regenerated from incomplete or noncanonical proof. This unit adds a pure typed eligibility gate, not GitHub mutation authority.

Scope

• Define a finite typed proof input representing canonical Units 1-6 completion and exact evidence identities.
• Return typed Pr130RegenerationBlocked for missing or mismatched proof.
• Return regeneration eligibility only when the proof is complete.
• Order block reasons deterministically.
• Treat frozen PR feat(app-builder): regenerate canonical Todo showcase #130 head d5928f99a122b04cb70ff68bfa02fa2d5648a5fc as an input expectation and evidence identity, never as a mutation target.
• Keep the gate pure, with no filesystem, network, GitHub read, or GitHub mutation capability.

Acceptance criteria

• The named test incomplete canonical proof blocks PR #130 regeneration before effects proves incomplete proof blocks before any mutation or read side effect.
• The named test mismatched evidence identities block PR #130 regeneration deterministically proves mismatches return ordered Pr130RegenerationBlocked reasons.
• The named test complete canonical proof returns regeneration eligibility only proves complete proof yields eligibility without performing regeneration.
• The named test frozen PR #130 head remains typed input evidence only proves the expected head remains d5928f99a122b04cb70ff68bfa02fa2d5648a5fc without reading or mutating GitHub.
• The gate never regenerates, updates, comments on, closes, rebases, or otherwise mutates PR feat(app-builder): regenerate canonical Todo showcase #130.
• The delivery stays within 400 changed lines (additions + deletions).
• Rollback removes only the eligibility gate and its tests.

Non-goals

• Regenerating PR feat(app-builder): regenerate canonical Todo showcase #130.
• Adding a GitHub SDK or client.
• Updating branches.
• Mutating showcase fixtures.
• Performing merge-chain actions.

Chain context

#143 (+ #145) → #147 → #149 → 📍 Unit 7 → frozen #130

Part of #131. This issue does not close prior issues.

[kattsushi]

Closing as not planned because Effectify App Builder has been retired in favor of contributing to lloydrichards/stack-effect. No App Builder implementation is retained on dev.