fix: static initialisers as constant expressions

[DavePearce]

This adds support for static initialises for static memory to be compile-time constant expressions, rather than just fixed numbers.

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Note

Medium Risk
Changes parsing/linking/typechecking/codegen for static memory contents and constant evaluation, which can affect program semantics and error reporting across the compiler pipeline. Risk is moderate due to new compile-time evaluation paths and overflow/underflow diagnostics.

Overview
Static memory initialisers are now parsed and stored as compile-time constant expressions (not just numeric literals). This threads expression-based Contents through AST (decl.Memory), parsing (parseStaticInitialiser), linking, and typing.

Codegen now evaluates static initialiser expressions at compile time via compileStaticInitialisers, and evalConstant was expanded to handle more operators (Sub/Div/Rem) while collecting syntax errors for overflow/underflow and cast overflow instead of panicking.

Tests and fixtures were updated/added to cover constant cycles, arithmetic overflow in constants/static initialisers, and static initialiser typing/unknown-symbol failures, plus a new unit test demonstrating constant-based static initialisation.

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