fix(build): accept GCC 12 for CUDA 12 compatibility

[Reiers]

Problem

The supraseal build.sh hard-requires GCC 13, creating an impossible build situation for CUDA 12.0–12.5 users:

1. Have CUDA 12 + GCC 12 → build.sh rejects GCC 12, demands 13 → ❌
2. Install GCC 13 → nvcc (CUDA 12.0–12.5) rejects GCC 13 as host compiler → ❌
3. No winning path without upgrading CUDA or skipping supraseal

Since supraseal now builds by default on Linux (PR #804), this blocks all CUDA 12.0–12.5 users from building Curio.

Root Cause

The GCC 13 requirement was a CI environment decision (PR #804 moved CI to Ubuntu 24.04 which defaults to GCC 13), not a code requirement. The supraseal codebase uses only C++17 features — fully supported by both GCC 12 and GCC 13.

CUDA / GCC Compatibility (per NVIDIA official docs)

CUDA Version	Max GCC Supported
CUDA 12.0–12.2	GCC 12.2
CUDA 12.6	GCC 13.2
CUDA 13.0+	GCC 13+

Changes

extern/supraseal/build.sh

• Accept both GCC 12 and GCC 13 (was: hard-require GCC 13 only)
• Prefer GCC 13, fall back to GCC 12
• Let nvcc validate the actual CUDA/GCC pairing via -ccbin

documentation/en/supraseal.md

• Document CUDA/GCC version pairing requirements
• Update GCC requirement from "GCC 13" to "GCC 12 or 13"

documentation/en/installation.md

• Update system requirements note for CUDA 12.x and GCC 12/13 pairing
• Add guidance on which GCC to install based on CUDA version
• Document DISABLE_SUPRASEAL=1 and FFI_USE_OPENCL=1 escape hatches

[Reiers]

Tested on Ubuntu Server 20.04, 22.04, and 24.04 — builds successfully on all three. GPUs show up correctly in the Curio web UI dashboard after the fix.