Require macOS 10.15 minimum so whisper.cpp builds for the universal macOS release #536

Target branch: release/0.7.0

Goal

The macOS release build sets an explicit minimum macOS version of 10.15 (Catalina), so whisper.cpp (speech input) compiles for aarch64-apple-darwin and the universal macOS build succeeds. The minimum is documented for users.

Context

• crates/speech depends on whisper-rs 0.16.0, which builds whisper.cpp through whisper-rs-sys 0.15.0.
• The universal macOS release build fails compiling whisper.cpp for aarch64-apple-darwin:
  • ggml/src/ggml-backend-reg.cpp:91:39: error: 'path' is unavailable: introduced in macOS 10.15
  • 'u8string' is unavailable: introduced in macOS 10.15 (std::filesystem)
• apps/smart-ssh-community/tauri.conf.json (bundle.macOS, ~line 44) sets no minimumSystemVersion, so Tauri's default 10.13 drives MACOSX_DEPLOYMENT_TARGET. std::filesystem needs 10.15.
• Tauri 2 itself documents macOS 10.15 (Catalina) as its minimum supported version, so this change only makes an existing requirement explicit; it drops no platform that was actually supported.
• .cargo/config.toml (added by Build whisper.cpp without native CPU flags (GGML_NATIVE=OFF) for release builds #534) has an [env] section with GGML_NATIVE = "OFF".
• README.md:76 (download section) lists the macOS .dmg without a minimum version. No spec lists supported OS versions.

Requirements

• Minimum macOS version is 10.15 (decided by maintainer).
• apps/smart-ssh-community/tauri.conf.json: add "minimumSystemVersion": "10.15" to bundle.macOS.
• .cargo/config.toml: add MACOSX_DEPLOYMENT_TARGET = "10.15" to the existing [env] section with a short comment (whisper.cpp uses std::filesystem, available from macOS 10.15; keeps plain cargo builds and build scripts such as the whisper-rs-sys CMake build on the same target as the bundle).
• README.md: state the minimum in the download section (e.g. macOS line: "ab macOS 10.15 (Catalina)"; keep the README's language). If README_DEV.md lists build/platform requirements, add the same minimum there.
• Changelog: a raised minimum OS version is user-visible. release/0.7.0 already has its changelog assembled (## [0.7.0] section in CHANGELOG.md), so add the entry directly to the ### Changed subsection of ## [0.7.0] in CHANGELOG.md (English, at most two lines, e.g. "macOS 10.15 (Catalina) or later is now required."), not as a fragment in changelog.d/.

Acceptance Criteria

• apps/smart-ssh-community/tauri.conf.json contains bundle.macOS.minimumSystemVersion: "10.15".
• .cargo/config.toml [env] contains MACOSX_DEPLOYMENT_TARGET = "10.15" with a short comment; GGML_NATIVE = "OFF" stays.
• On macOS: cargo build --release --target aarch64-apple-darwin -p <speech crate> compiles, and target/**/build/whisper-rs-sys-*/out/**/CMakeCache.txt contains CMAKE_OSX_DEPLOYMENT_TARGET = 10.15 (or the build output shows -mmacosx-version-min=10.15). Without macOS available: both config values verified by inspection and stated in the PR description.
• README.md states macOS 10.15 (Catalina) or later as the minimum.
• CHANGELOG.md section ## [0.7.0] contains the entry about the macOS 10.15 minimum; no new file in changelog.d/.
• CI is green.

Technical Notes

• Not a security-critical area; no data model change. No workflow change.
• An explicit MACOSX_DEPLOYMENT_TARGET in a build environment wins over Cargo [env] (no force = true); the Tauri bundler sets it from minimumSystemVersion, which then matches.
• [env] values apply on every host; MACOSX_DEPLOYMENT_TARGET is ignored outside Apple targets.

Spec impact: none

Related Work

• Build whisper.cpp without native CPU flags (GGML_NATIVE=OFF) for release builds #534 (added .cargo/config.toml)