Make repo release-ready: add sym-lib-table, CHANGELOG, expand .gitignore

- Add project-local sym-lib-table so the custom esp_sensair_stuff symbols
  resolve on a fresh clone (fp-lib-table already covered footprints)
- Add CHANGELOG.md documenting the v1.0.0 release
- Expand .gitignore: KiCad lock files, ESPHome build artifacts, Python
  cache, editor/OS noise, logs, merge artifacts
- README: list sym-lib-table and CHANGELOG.md in the repository layout

README.md

@@ -168,11 +168,13 @@ ESPHome on `esp32-c3-devkitm-1`, framework **esp-idf**. Config template:
 ├── esp_sensair_stuff.kicad_sym  custom symbols (ESP32-C3, Sensirion S8, BME280)
 ├── esp_sensair_stuff.pretty/    custom footprints, drawn from component datasheets
 ├── fp-lib-table                 project-local footprint library table (${KIPRJMOD})
+├── sym-lib-table                project-local symbol library table (${KIPRJMOD})
 ├── gerber/                      Gerber X2, Excellon drills, job file, ready-to-upload zip
 ├── bom/                         bill of materials (CSV)
 ├── docs/                        schematic PDF, layer plots, 3D render
 ├── firmware/                    ESPHome config templates, secrets templates, notes
 ├── photos/                      photographs of the assembled board
+├── CHANGELOG.md                 release history
 ├── LICENSE                      CERN-OHL-S-2.0 (hardware)
 └── LICENSE.MIT                  MIT (firmware config, BOM, docs)
 ```