Mock provider: integer literals overflowing i64 silently become 0 #74

Audit finding M8 — verified live.

Problem

puzzle/lex.rs read_number falls back to 0 on overflow. target = 99999999999999999999 lexes as 0 with no diagnostic, and the compiled artifact carries "target":0 — silent corruption of a normative output.

Acceptance criteria

• Out-of-range integer literals produce a diagnostic (and a defined value policy), never a silent 0.
• Regression test + green suite.