## Problem
Fixes#743 — High memory usage / OOM with relatively small dataset.
`trackedBytes` severely undercounted actual per-packet memory because it
only tracked base struct sizes and string field lengths, missing major
allocations:
| Structure | Untracked Cost | Scale Impact |
|-----------|---------------|--------------|
| `spTxIndex` (O(path²) subpath entries) | 40 bytes × path combos |
50-150MB |
| `ResolvedPath` on observations | 24 bytes × elements | ~25MB |
| Per-tx maps (`obsKeys`, `observerSet`) | 200 bytes/tx flat | ~11MB |
| `byPathHop` index entries | 50 bytes/hop | 20-40MB |
This caused eviction to trigger too late (or not at all), leading to
OOM.
## Fix
Expanded `estimateStoreTxBytes` and `estimateStoreObsBytes` to account
for:
- **Per-tx maps**: +200 bytes flat for `obsKeys` + `observerSet` map
headers
- **Path hop index**: +50 bytes per hop in `byPathHop`
- **Subpath index**: +40 bytes × `hops*(hops-1)/2` combinations for
`spTxIndex`
- **Resolved paths**: +24 bytes per `ResolvedPath` element on
observations
Updated the existing `TestEstimateStoreTxBytes` to match new formula.
All existing eviction tests continue to pass — the eviction logic itself
is unchanged.
Also exposed `avgBytesPerPacket` in the perf API (`/api/perf`) so
operators can monitor per-packet memory costs.
## Performance
Benchmark confirms negligible overhead (called on every insert):
```
BenchmarkEstimateStoreTxBytes 159M ops 7.5 ns/op 0 B/op 0 allocs
BenchmarkEstimateStoreObsBytes 1B ops 1.0 ns/op 0 B/op 0 allocs
```
## Tests
- 6 new tests in `tracked_bytes_test.go`:
- Reasonable value ranges for different packet sizes
- 10-hop packets estimate significantly more than 2-hop (subpath cost)
- Observations with `ResolvedPath` estimate more than without
- 15 observations estimate >10x a single observation
- `trackedBytes` matches sum of individual estimates after batch insert
- Eviction triggers correctly with improved estimates
- 2 benchmarks confirming sub-10ns estimate cost
- Updated existing `TestEstimateStoreTxBytes` for new formula
- Full test suite passes
---------
Co-authored-by: you <you@example.com>
