Rename project esp+sensair -> esp-sensair; publish fabrication files as release assets

- Rename KiCad project/schematic/board (and local .prl) to drop the '+',
  updating all internal cross-references (project name, sheetfile, title blocks)
- Remove the '+' from Gerber/drill/job filenames and the job file's internal
  Name/Path references; rebuild the upload zip with clean names
- Stop tracking gerber/ (derived artifacts): untrack, gitignore, and publish
  the fabrication zip as a GitHub Release asset instead
- README/CHANGELOG: point fabrication data at the release, mark the '+' naming
  issue as fixed, refresh the repository layout

CHANGELOG.md

@@ -14,8 +14,9 @@ First public release of the ESP32-C3 CO₂ & environment sensor board (prototype
   (`esp_sensair_stuff.pretty/`) for the ESP32-C3 module, Sensirion S8 and BME280.
 - Project-local library tables (`fp-lib-table`, `sym-lib-table`) so the design
   opens with all custom parts resolved on a fresh clone.
-- **Fabrication data** in `gerber/`: Gerber X2 plot, Excellon PTH/NPTH drills,
-  Gerber job/stackup file, and a ready-to-upload `esp-sensair.zip`.
+- **Fabrication data**: Gerber X2 plot, Excellon PTH/NPTH drills, Gerber
+  job/stackup file, packaged as a ready-to-upload `esp-sensair.zip`. These are
+  derived artifacts published as GitHub Release assets (not committed to the tree).
 - **Bill of materials** (`bom/esp-sensair-bom.csv`) plus the human-readable BOM
   in the README.
 - **Documentation** in `docs/`: schematic PDF, top/bottom layer plots, 3D render.
@@ -32,5 +33,5 @@ First public release of the ESP32-C3 CO₂ & environment sensor board (prototype
 See the "Known issues and lessons learned (v1 → v2)" section of the
 [README](README.md). Highlights: power delivered through the module's USB-C 5 V
 path, single decoupling capacitor, no mounting holes, no populated I²C pull-ups,
-unannotated schematic, leaded HASL, and a `+` in the project name that leaks
-into Gerber file names.
+unannotated schematic, and leaded HASL. (The `+` that was in the project name has
+been removed; the project and fabrication files are now `esp-sensair`.)