Hardening: Adopt safe buffer models, bounds-checked roots, and verifier pointer safety

[jagadish-chandra-vantimi]

Summary of Changes

This patch introduces proactive memory-safety hardening and safe buffer abstractions across core FlatBuffers parsing and verification infrastructure:

1. FlexBuffers Root Width Validation (flexbuffers.h):

   • Enforce valid power-of-two scalar byte widths (1, 2, 4, 8) and buffer size sufficiency during root extraction to prevent out-of-bounds reads and stack buffer inconsistencies on malformed inputs.

2. Pointer Range & Relational Comparison Safety (verifier.h):

   • Implement uintptr_t total-ordering pointer range checking (CheckPointerInRange) across verifier entrypoints to prevent undefined behavior in pointer relational operations and avoid pointer wrap hazards.
   • Fix arithmetic in size-prefixed buffer verification (size_ - sizeof(SizeT)) to prevent integer overflow wrapping.
   • Enforce structural vtable minimums (vsize >= 2 * sizeof(voffset_t)), cleanly rejecting degenerate tables.
   • Propagate depth and table traversal budgets across nested FlatBuffers verification to prevent excessive recursion.

3. Secure-by-Design Verification APIs (verifier.h):

   • Introduce canonical GetVerifiedRoot<T> and GetVerifiedSizePrefixedRoot<T> in flatbuffers/verifier.h so consuming applications can verify and deserialize buffer roots in a single atomic call.

4. Table Offset Validation (table.h):

   • Prevent table metadata field misinterpretation by rejecting queries for internal metadata offsets (offsets < 4) and unaligned field indices in GetOptionalFieldOffset.

5. Safe Buffer Model & Bounds-Checked Accessors (vector.h, array.h, stl_emulation.h, buffer_ref.h):

   • Add GetOptional(i), GetSafe(i, default), and runtime bounds-checked MutateSafe(i, val) to vectors and arrays.
   • Provide first-class span integration across detached buffers, buffer references, and root accessors.

6. Automated Testing (tests/test.cpp):

   • Add MemorySafetyHardeningTest covering all boundary conditions, malformed header rejections, safe mutation routines, and verified root extraction.

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