ttl: resolve and type-check imports across multi-file ttl modules

[kdy1]

Summary

Imports are parsed but always surfaced as unsupported_imports, so multi-file module symbol resolution is unavailable.

Evidence

Contract Evidence:

• docs/project-ttl-language.md:44
• cmds/ttlc/internal/sema/checker.go:45

Current Gap

Imports are parsed but always surfaced as unsupported_imports, so multi-file module symbol resolution is unavailable.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Resolve import paths within allowed workspace boundaries.
• Build module-level symbol tables across imported files.
• Type-check cross-file references and emit precise diagnostics for unresolved imports/symbols.
• Remove unsupported-import diagnostics for valid import usage.

Test Scenarios

• Successful multi-file module compile with imported task/type references.
• Unresolved import path yields deterministic error diagnostics.
• Imported symbol mismatch yields type diagnostics with accurate source location.

Out of Scope

• External package registries.
• Remote dependency fetching.