Centralize overflow-safe layout sizing and reserve validation

[jmestwa-coder]

Summary

Centralize overflow-safe layout sizing and reservation logic across layout implementations.

Changes

• Added shared internal helpers in layout_priv.h for:

  • saturating layout size doubling
  • guarded reserve sizing validation

• Replaced unchecked sizing arithmetic in:

  • FMTLayout
  • HTMLLayout
  • JSONLayout
  • PatternLayout
  • XMLLayout

• Replaced direct sizing operations such as:

expectedPatternLength * 2
output.reserve(a + b)

with centralized checked helpers.

Why

The previous layout sizing logic used unchecked size_t arithmetic during:

• expected buffer growth calculation
• reserve size computation

This change avoids invalid reserve calculations and overflow-prone sizing behavior while preserving existing formatting semantics.

Behavior

• No formatting or output changes
• No parser or layout behavior changes
• Only affects preallocation sizing paths
• Falls back to normal append growth when reservation would exceed valid bounds

Tests

Added deterministic coverage for:

• saturation behavior
• boundary arithmetic cases
• successful reserve paths
• overflow/skip reserve paths

Validated with focused jsonlayouttest coverage.