pkg/normalizer: populate function name for v2 arrow locations

[gnurizen]

Kernel frames sent through the v2 Arrow write path appear as bare addresses or <unknown> in the flame graph, although the agent sends the symbol.

Cause

The v2 schema has only system_name on the function struct; the agent puts the kernel symbol there. encodeV2Location wrote the function name as "". The flame graph (flamegraph_arrow.go) and table read only function_name, so for every pre-symbolized v2 frame:

• sibling frames merge into one node keyed on the empty name, and location_address goes null wherever the merged addresses differ
• the UI's nodeLabel falls back to [mapping]0x<addr>, where the address is module-relative

Fix

• Ingest (pkg/normalizer/arrow_v2.go): write system_name as both the name and system_name. Query-time demangling already runs over the name.
• Decode (pkg/profile/decode.go): DecodeInto and DecodeFunctionName fall back to system_name when the name is empty, so profiles already stored by the old v2 path display correctly (FrostDB). Once that data ages out of retention, the fallback can be removed.

Testing

• TestAddSampleRecordV2 now asserts that the encoded location carries the symbol as both name and system name.
• New TestDecodeFallsBackToSystemName covers the legacy encoding.
• With the fixes stashed, both tests fail; with them in place, both pass.
• pkg/profile, pkg/normalizer, pkg/parcacol and pkg/query tests pass.
• Not yet checked end to end against a live agent.

[alwaysmeticulous]

✅ Meticulous spotted 0 visual differences across 282 screens tested: view results.

Meticulous evaluated ~4 hours of user flows against your PR.

_{Expected differences? Click here. Last updated for commit dd5fcd5 pkg/normalizer: populate function name for v2 arrow locations. This comment will update as new commits are pushed.}

[brancz]

whops