ttl: enforce core-type and read(vc) semantic validation

[kdy1]

Summary

Current semantic checks cover return shape and read arity, but do not enforce comprehensive core-type contracts or read(vc) type boundaries.

Evidence

Contract Evidence:

• docs/project-ttl-language.md:44
• docs/project-ttl-language.md:99
• cmds/ttlc/internal/contracts/enums.go:36
• cmds/ttlc/internal/sema/checker.go:150

Current Gap

Current semantic checks cover return shape and read arity, but do not enforce comprehensive core-type contracts or read(vc) type boundaries.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Enforce stable rules for TTL core types (Vc, ResolvedVc, OperationVc, TransientValue, State) as applicable to phase scope.
• Validate read(...) input/output typing against expected Vc-based contracts.
• Emit dedicated diagnostics for core-type misuse and invalid read usage.
• Keep behavior deterministic and documented.

Test Scenarios

• Valid read(vc) usage succeeds with no diagnostics.
• Non-Vc read input yields semantic error.
• Core-type misuse surfaces targeted diagnostic kind/message.

Out of Scope

• New language syntax unrelated to core type semantics.
• Runtime-side dynamic type checks beyond compile contract.