ci: add a separate lane for heavy type-checking tests

[martyy-code]

Description

Add a dedicated CI lane for exhaustive conditional type tests, deep recursive type checks, and full monorepo type validation. This lane runs on merge to main only (not on every PR) to keep the fast feedback loop intact.

Why

Type-level tests can be expensive. Exhaustive conditional types, deep recursive checks, and full-project type validation take seconds to minutes. Running them on every PR slows down the fast path. The 2026 type-testing playbook recommends separating these into a dedicated lane.

Acceptance criteria

• CI has a "deep-check" lane that runs on merge to main
• Lane includes: exhaustive conditional tests, recursive deep checks, full attw
• Failure on main blocks the release train
• Documented in CONTRIBUTING.md

Risks

• Low. Turbo remote cache keeps this fast after first run.

Related

• chore: adopt TypeScript 7 as baseline and document multi-version compatibility #47 (adopt TS7 as baseline) — TS7's 10x speed makes this even cheaper