Fixes#1849.
## Problem
The packets table "HB" (Hop Bytes / col-hashsize) column always shows
`1` for TRACE packets. TRACE packets use header path bytes as per-hop
SNR readings, not truncated hop hashes — see
`internal/packetpath/route.go` `PathBytesAreHops(TRACE) = false`. The
high-2-bit "hash_size" derivation applied to a SNR byte is meaningless
(typically `1`).
## Fix
At the 3 render sites in `public/packets.js` (`buildGroupRowHtml` header
row, its child rows, `buildFlatRowHtml`), when `payload_type === 9`
(TRACE) render `—` in the `col-hashsize` cell with a `title` tooltip
explaining that TRACE path bytes are SNR readings and directing users to
the sidebar decoder for the actual hop count. Non-TRACE rows unchanged.
## TDD
- Red commit: `dd7a4e78` — `test-issue-1849-trace-hashbytes.js` asserts
col-hashsize cell equals `—` with a title tooltip for TRACE, numeric for
non-TRACE. CI must fail on this commit.
- Green commit: `115368d0` — 3-site fix in `public/packets.js`.
## Verification
```
$ node test-issue-1849-trace-hashbytes.js
✅ All 4 tests passed
```
Preexisting failures in `test-packets.js` (13) are unchanged by this PR
(verified via `git stash`) — unrelated to the surface touched here.
## Scope
- `public/packets.js`: 3 render sites, +21/-8.
- `test-issue-1849-trace-hashbytes.js`: new unit test.
- `test-all.sh`: wire new test.
No public API change.
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Co-authored-by: clawbot <bot@example.invalid>
