[xiaods]

fix(sandboxd): report the changed entry's name in watch events

Running the K8E guide's Level 4 example against the cluster showed the
watch event carrying the watched directory instead of the file that
changed:

[PASS] Level 4 — files.watch_dir (polling trio):
  guide form -> TypeError (guide snippet bug)
  get_new_events() -> [('/workspace//', 'FilesystemEventType.CREATE')]

The guide expects event.name to be "a.txt".

Two bugs, both in sandboxd/src/watch.zig:

1. The inotify record was mis-parsed. inotify_event.len is the length
   of the NAME — including its terminating NUL, padded so the record
   stays 4-byte aligned — not the length of the record. The code treated
   it as the record length, so:

   • the name slice was computed as buf[pos+16 .. pos+len-17], which
     collapses to the empty string for any name of 15 bytes or less.
     "a.txt" is 5 bytes, so every event reported an empty name;
   • the cursor advanced by len instead of by the record length, which
     desynchronised the event stream after the first record.

   The record length is header + len, and the name ends at the first
   NUL — len counts the NUL and the alignment padding, so the name is
   not len-1 bytes long.

2. The event name was the joined absolute path. The SDK documents
   FilesystemEvent.name as the "relative path to the filesystem object"
   — relative to the watched directory — which is exactly what inotify
   hands over as the entry name. The SDK matches the name against the
   entries it listed in that directory, so the absolute path was wrong
   twice over, and the join produced a doubled slash when the name was
   empty.

The parsing is now a pure function, parseInotifyRecords(), so it can be
unit-tested without an inotify fd.

Also: sandboxd/src/watch_test.zig was never wired into zig build test
— build.zig listed exec/files/transcript/events/pty/httpio but not
watch, so the two watch tests that existed had never run. Adding the
module brings the file in and the four new cases with it.

Verified against the guide's own Level 4 example:

before: get_new_events() -> [('/workspace//', 'CREATE')]
after:  get_new_events() -> [('a.txt', 'CREATE'), ('a.txt', 'WRITE')]

sandboxd: 67/67 tests pass (was 61: 4 new watch cases + the 2 that
had never been collected). The four new cases were checked against the
old parser — three of them fail without the fix.

Full E2B SDK regression on the cluster: Python 44/44, JavaScript 17/17.
The guide's Level 1-5 examples pass on both languages.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

1 024 tests  ±0   997 ✅ ±0   6m 59s ⏱️ -6s
  120 suites ±0    27 💤 ±0 
    1 files   ±0     0 ❌ ±0 

Results for commit 85bb59a. ± Comparison against base commit 2ed56f1.