release: prepare v1.0.0-rc3

[Aybook]

Summary

Bumps `_VERSION` 1.0.0-dev -> 1.0.0-rc3 in `core/const.lua` and updates `.github/release-body.md` for the upcoming `v1.0.0-rc3` tag.

What rc3 ships over rc2

• PR fix(gui): tray exit crash + restore from tray (closes #61) #64 (GUI: tray exit crashes with wxWindowBase assertion (any pushed event handlers must have been removed) #61) - tray exit crash + restore-from-tray
• PR fix(gui): TLS RadioBox overflow + taskbar icon + About refresh (closes #63) #65 (GUI: TLS RadioBox text overflow + missing taskbar icon (wxLua 3.x regressions) #63) - TLS RadioBox overflow + branded Windows .exe icon + About refresh
• PR fix(gui): capture worker stderr at process exit (closes #59) #66 (GUI worker stderr is lost (wxProcess:Redirect with unread stream) #59) - GUI worker stderr capture into `log/logfile.txt`
• PR feat(certs): auto-generate TLS cert on first run (closes #62) #67 (Auto-generate TLS cert on first run if none present #62) - first-run TLS cert auto-generation

Test plan

• Lua 5.4 syntax check via `lua54 loadfile core/const.lua` -> OK
• All four feature PRs already merged + their own CI green pre-merge
• CI smoke green on this prep PR (Linux + Windows)

Once merged, tag `v1.0.0-rc3` will be pushed on main; `release.yml` auto-detects the `-rc3` suffix as a SemVer pre-release via `contains(github.ref_name, '-')`.

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