chore: C99/C11 modernization for readability

[Liam-Deacon]

Problem

Large parts of the codebase are written in a C89/C90 style (e.g., loop-scope variables declared at function scope, manual index manipulation, macro-heavy helpers). This makes the code harder to read and reason about, and it limits the use of clearer C99/C11 constructs.

Proposed direction

Adopt C99 as the project baseline (and selectively use C11 where it improves clarity) to modernize code style and reduce macro reliance.

Examples (non-exhaustive)

• Loop-scope variables: prefer for (size_t i = 0; i < n; i++) over function-scope size_t i; + loop reuse. This removes accidental reuse and keeps variable lifetimes tight.
• Safer/clearer helpers: replace macro-heavy constructs (e.g., MIN/MAX, assertion macros with side effects) with static inline functions. This improves type safety and debuggability.
• stdbool/size_t: use <stdbool.h> for bool and <stddef.h>/size_t for sizes instead of ad‑hoc int/unsigned long usage to reduce format/overflow mistakes.
• Designated initializers: prefer { .field = value } for clarity in large structs.
• Explicit casts and enums: tighten enum usage and avoid implicit int assumptions.

Benefits

• Better readability and maintainability (fewer hidden macro tricks).
• Safer, more explicit code with tighter variable scopes.
• Easier static analysis and fewer false positives from linters.
• Simplified onboarding and contribution (modern C expectations).

Acceptance criteria

• Documented C99/C11 baseline (CMake + Autotools) and scope of modernization.
• Initial pass converts a small, representative subset of files with clear before/after examples.
• No functional regressions (build + existing tests pass).

Notes

• Keep changes incremental (file-by-file) to avoid huge PRs.
• Be conservative with C11 features; only use them where they clearly improve readability or safety.