fix(macos): support macOS Tahoe (15+) SDK build

[claudiojara]

Summary

Two minimal fixes so local macOS builds work on macOS 15+ (Sequoia/Tahoe) with current Xcode SDKs. Plus a small helper script for local DMG packaging.

What breaks today

On a clean macOS 15+ checkout, ./build_release_macos.sh -d -a arm64 fails twice:

1. Autotools deps cannot find libSystem

GMP / MPFR are built via cmake ExternalProject_Add which forks configure — autotools does not inherit CMAKE_OSX_DEPLOYMENT_TARGET, and the macOS 15+ SDK requires SDKROOT to be set explicitly:

ld: library 'System' not found
cc: error: linker command failed with exit code 1
configure: error: could not find a working compiler

Fix (build_release_macos.sh): auto-export SDKROOT="$(xcrun --show-sdk-path)" when unset. Harmless on older SDKs (xcrun returns the path regardless).

2. clang on SDK 26 rejects constexpr enum-cast in wxMediaCtrl2.h

// src/slic3r/GUI/wxMediaCtrl2.h:41
static constexpr wxMediaState MEDIASTATE_BUFFERING = (wxMediaState) 6;
//                                                  ^ value 6 is outside [0,3]

Older clang accepted the out-of-range cast in a constexpr context. Current clang (with the macOS 15+ SDK) rejects it: constexpr variable 'MEDIASTATE_BUFFERING' must be initialized by a constant expression.

Fix (wxMediaCtrl2.h): mirror the existing working pattern in src/slic3r/GUI/MediaPlayCtrl.h:74 — static inline const wxMediaState ... = static_cast<wxMediaState>(6). Identical runtime behavior, no constexpr requirement.

Extras

• .gitignore: ignore .tools/ (project-local CMake), dist/ (downloaded releases), tools/pjarczak_bambu_linux_host/runtime/ (generated output)
• package_macos_dmg.sh (new): wraps hdiutil to produce an unsigned DMG from a prebuilt .app, mirroring the CI flow. Auto-detects arch (arm64/x86_64/universal), reads version from version.inc, strips com.apple.provenance xattrs that confuse hdiutil on macOS 15+. Pure dev-side helper; safe to drop if not desired.

Verified

• Built locally on macOS 15 Apple Silicon with Xcode 26.5 SDK (deps in 12m42s, slicer in 20s incremental)
• Re-ran build_macos_bridge.yml on my fork (https://github.com/claudiojara/OrcaSlicer-bambulab/actions/runs/25776649189) — all four jobs passed, produced 237 MB universal DMG identical in size to the v1.0.0 release artifact
• No behavior change on Linux/Windows paths (build_release_macos.sh is macOS-only; wxMediaCtrl2.h change is functionally equivalent)

Test plan

• Maintainer reviews diff (16 insertions, 1 deletion across 3 tracked files + 1 new helper)
• CI run on this branch passes
• Local build attempt on macOS 15+ Apple Silicon succeeds without manual env wrangling

🤖 Related debugging session also surfaced an Apple Silicon runtime issue with pjarczak_bambu_linux_host that is unrelated to these build fixes; filing a separate issue with the stack trace.

[claudiojara]

Suggested label (couldn't add as external contributor): bug.

[seuros]

The fact that you used Claude to ship this PR is funny. Now Bambulab can sue Anthropic.

This PR is correct. 🚢 it.