Source linked list uses 16-bit pointers — potential truncation like #473

[thedavidmeister]

Description

newActiveSourcePointer in LibParseState.sol packs a memory pointer into 16 bits:

• Line 224: shl(0x10, oldActiveSourcePointer) — stores pointer in bits 16+
• Line 228: and(..., 0xFFFF) to store new pointer in low 16 bits
• Line 495: shr(0xf0, mload(itemSourceHead)) — reads linked list next pointer

This is the same 16-bit pointer pattern that caused #473 in the sub-parser linked list. In practice, sources are allocated sequentially during a single parse so memory growth is bounded, but the constraint is implicit rather than enforced.

Should either:

1. Widen the pointer bits (as done for sub-parsers in Fuzz failure: LibParseStateExportSubParsersTest #473), or
2. Add an explicit check/comment documenting why 16 bits is safe for this specific use case

The shift should also use a named constant rather than magic numbers.

[thedavidmeister]

Fixed in PR #477 — sub-parser linked list pointers now use 96 bits.