[Core] Configurable aliasing (IAliasProvider)

[jeffpatton1971]

Summary
Extract alias mapping (e.g., System.Int32 -> int) into IAliasProvider and use consistently in signatures and anchors.

Tasks

• Define IAliasProvider + DefaultAliasProvider.
• Replace hard-coded aliasing with provider calls.
• Tests for token-aware aliasing (avoid breaking identifiers like StringComparer).

Acceptance

• Snapshot parity with defaults; custom maps honored.