refactor(scripts): extract gate-common.mjs, decompose verify-pr-checks + verify-no-control-bytes

[dean0x]

Summary

Consolidate release-gate tooling by extracting common Result/validation patterns from verify-pr-checks.mjs and verify-no-control-bytes.mjs into a shared scripts/lib/gate-common.mjs. Decompose the two 224-line main/evaluateChecks functions.

Deferred Issues

From review [wave-v0.4.0-wave1/2026-08-19_1216]:

Finding	File:Line	Reason
architecture-05	scripts/verify-no-control-bytes.mjs:530	Two-file extraction + test-harness rewrite on release-gate tooling.
architecture-09	scripts/verify-no-control-bytes.mjs:467	Rewriting the scanner core of a live security gate.
complexity-05	scripts/verify-pr-checks.mjs:218	Decomposing the function that gates every merge, days before an irreversible tag.
complexity-06	scripts/verify-no-control-bytes.mjs:530	Exporting main would rewrite 33 of 46 spec tests.

Acceptance Criteria

• Extract scripts/lib/gate-common.mjs with Result types and validation utilities
• Refactor verify-pr-checks.mjs to decompose 224-line main function
• Refactor verify-no-control-bytes.mjs to decompose 224-line evaluateChecks function
• All 46 spec tests pass with new decomposition
• Both scripts maintain exact output compatibility

Related

• PR v0.4.0 Wave 1: breaking changes, live-bug fixes, and release-safety gates #308 (wave/v0.4.0-wave1)
• Release-gate quality infrastructure

For v0.5.0