M0: label local builds in --version, About and the package release (#1529)

A build compiled with PDFCRAFT_BUILD_LABEL=<label> shows "0.7.1 (<label>)"
in `pdfcraft --version`, `pdfcraft-cli --version`, Help > About and the
first log line (pdfcraft_engine::build_info). The version itself is
unchanged, so the update check, the MCP server info and the User-Agent
still carry the release version.

packaging/linux/package.sh puts the label in the file names
(pdfcraft-0.7.1-<label>-linux-x86_64.rpm) and makes the .rpm/.deb release
`1.<label>` (nfpm `release: ${RELEASE}`, 1 when unlabelled). rpm sorts
0.7.1-1.<label> after 0.7.1-1 and before 0.7.2-1, so a locally built
package upgrades over the installed release and the next release replaces
it. packaging/env.sh accepts only letters, digits and inner dots, which
are valid in both rpm and deb release fields.

Unlabelled builds are unchanged.

Made with Claude Code (Claude Opus 5.5, medium effort). It passes the
workspace quality gates (fmt, clippy, cargo test --workspace, xtask assets)
and was checked by hand on Fedora 44, but it has not been exhaustively
regression tested.

Co-authored-by: bmh129 <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/development.md

@@ -115,5 +115,6 @@ driver's shader compilation, and explicit `PDFCRAFT_RENDERER=wgpu` still disable
 | `WGPU_POWER_PREF` | GPU choice; by default PdfCraft draws on the GPU the Wayland compositor draws with (it says so through linux-dmabuf feedback), else the GPU that drives the primary display (Windows) or the built-in panel (Linux), else the low-power (integrated) GPU. The log says which one was chosen |
 | `WGPU_BACKEND` | Graphics backend; by default Windows uses Direct3D 12, falling back to OpenGL |
 | `PDFCRAFT_RENDERER` | `gl` starts with OpenGL (glow) and never loads wgpu; `wgpu` reports a wgpu failure instead of retrying with OpenGL; unset, wgpu is retried with OpenGL when it can't start (see [Renderer fallback](#renderer-fallback)) |
+| `PDFCRAFT_BUILD_LABEL` | Build time: a label for a local or patched build (letters, digits, inner dots), shown after the version in `--version`, About and the log (`0.7.1 (titlebar)`). `packaging/linux/package.sh` also puts it in the file names and the .rpm/.deb release (`1.titlebar`), which upgrades over the release of the same version |
 | `CRAFT_FONTS_DIR` | Build time: a [craft-fonts](https://github.com/storytold/craft-fonts) checkout to embed (Japanese fonts) |
 | `PDFCRAFT_SYSTEM_FONTS` | `0` stops the desktop app from using an installed font for characters its embedded fonts lack (`cargo xtask screenshots` sets it) |