Vendor the ASN.1 modules the codecs are verified against; untrack IDE churn

asn1/
Three tests assert exact bytes - CamEncodeGoldenTest, DenmAirReceiveTest and
SpatemUperCodecTest - and their expected values came from asn1tools compiled
against ETSI modules that existed only as an untracked working copy on one
machine. A golden-byte fixture nobody else can regenerate is a fixture nobody
can safely touch, so the modules are now in the repo.

Only the seven .asn files those tests need are copied, 576 KB of a 4.2 MB
checkout; the upstream Rust parser is not used by this project at all. Verified
sufficient in isolation: copied into an empty directory, all three specs compile
and reproduce the committed golden CAM bytes byte-identically.

Source is consider it GmbH's C-ITS-Parser (github.com/consider-it/C-ITS-Parser)
at f457426, MIT licensed - LICENSE is retained alongside as that requires. The
schemas themselves are ETSI's standard definitions; upstream's contribution is
assembling them into a compilable set. asn1/README.md records the provenance,
which module pairs with which message, and the rule that matters: never
regenerate a golden fixture from this project's own encoder, because sharing a
mistake between encoder and decoder is exactly the failure these files exist to
catch.

Doc references in the codecs and tests now point at asn1/ instead of the
untracked checkout, and C-ITS-Parser/ is gitignored so the working copy beside
the project is never picked up.

Untracked local state
- .idea/deploymentTargetSelector.xml rewrites itself on every deploy, so it has
  been showing as modified in essentially every commit. Along with
  deviceManager.xml, appInsightsSettings.xml and studiobot.xml it is per-machine
  state, not project configuration.
- obu-firmware/sdkconfig.old is ESP-IDF build output - it is the previous
  sdkconfig, rewritten on every build. sdkconfig.defaults remains tracked, since
  that is the configuration actually chosen.

All five stay on disk; only the tracking is removed. Also ignores
.claude/settings.local.json, which is per-machine, while leaving the skills
beside it committable as project knowledge.

.gitignore

@@ -17,3 +17,24 @@ local.properties
 *.jks
 *.keystore
 secrets.properties
+
+# --- local IDE state, not project configuration -------------------------------
+# Which device you last deployed to, your emulator list, and Studio's assistant
+# settings are per-machine. deploymentTargetSelector.xml in particular rewrites
+# itself on every run, so tracking it means a dirty tree after every deploy.
+/.idea/deploymentTargetSelector.xml
+/.idea/deviceManager.xml
+/.idea/appInsightsSettings.xml
+/.idea/studiobot.xml
+
+# ESP-IDF rewrites sdkconfig on every build and keeps the previous one here.
+# sdkconfig.defaults is the real, intentional configuration; these two are output.
+sdkconfig.old
+
+# Claude Code local settings (permissions, per-machine). The skills alongside it
+# are project knowledge and may be committed deliberately.
+.claude/settings.local.json
+
+# Third-party working copies kept beside the project, not part of it. The ASN.1
+# modules this project actually needs are vendored under asn1/ instead.
+/C-ITS-Parser/