Add Python 3.14 support and update CI infrastructure

[Copilot]

Adds Python 3.14 support across all platforms and modernizes CI to use latest macOS runners, following patterns from lensfunpy#64.

Changes

Python 3.14 matrix entries

• Added to all platforms (Linux x86_64/aarch64, macOS x86_64/arm64, Windows x86_64) with numpy 2.3.*

macOS runner updates

• x86_64: macos-13 → macos-15-intel
• arm64: macos-14 → macos-15

Dynamic Python path resolution
Replaced hardcoded version conditionals with computed paths:

# Before: manual if/elif chain for each version
if [ $PYTHON_VERSION == "3.13" ]; then
    PYBIN="/opt/python/cp313-cp313/bin"
elif...

# After: automatic computation
PYVER_NO_DOT=${PYTHON_VERSION//./}
PYBIN="/opt/python/cp${PYVER_NO_DOT}-cp${PYVER_NO_DOT}/bin"

Build optimization

• Added PIP_PREFER_BINARY=1 across all build scripts
• Added pip install --upgrade pip before dependency installation

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