fix(telemetry): drop undocumented ext.os.release / ext.app.initTs

[timenick]

Closes #635.

Diagnosis

The OneCollector backend rejected every batch with
{"acc":0,"efi":{"InvalidEventFormat":"all"}}. A live probe against
the production ingest narrowed the cause to two fields in the
envelope that are not part of the documented CS 4.0 extension
slots — either field on its own is enough to flip the whole batch
from acc:1 to InvalidEventFormat: all:

envelope field	CS 4.0 status
ext.os.release	not documented (ext.os keys: name, ver, bootId)
ext.app.initTs	not documented (ext.app keys: name, ver, id, iid, expId, userId, sesId, env, asId, locale, tz)

ext.app.initTs is rejected for every value type (float, int, ISO
8601 string), so this is the field name being unknown to the
schema, not a type mismatch.

The previous acc:1 validation in #456 happened to use a minimal
envelope (no ext), so the bad fields never reached the wire. The
existing unit tests asserted the envelope shape but never validated
it against the live backend, so the regression slipped through.

Fix

• _resource_to_ext() no longer maps os.release → ext.os.release
  or initTs → ext.app.initTs.
• _build_resource() no longer puts those attributes in the
  Resource at all; the _init_ts instance attribute is dropped too
  (it was only read by the now-removed mapping).
• The "translates resource to ext" unit test is updated to assert
  the trimmed envelope, plus a new test_export_omits_undocumented_cs40_ext_fields
  fails fast if anyone re-introduces either mapping.

Probe log (production ingest, masked)

[01 minimal Heartbeat (control)]                              status=200  body={"acc":1}
[02 Heartbeat + full ext (initTs FLOAT)]                      status=400  body={"acc":0,"efi":{"InvalidEventFormat":"all"}}
[03 Heartbeat + full ext (initTs STRING)]                     status=400  body={"acc":0,"efi":{"InvalidEventFormat":"all"}}
[07 Heartbeat + only ext.app.initTs as FLOAT]                 status=400  body={"acc":0,"efi":{"InvalidEventFormat":"all"}}
[14 ext.device only {localId, authId, deviceClass}]           status=200  body={"acc":1}
[15 ext.os only {name, ver, release}]                         status=400  body={"acc":0,"efi":{"InvalidEventFormat":"all"}}
[16 full ext WITHOUT initTs but WITH release]                 status=400  body={"acc":0,"efi":{"InvalidEventFormat":"all"}}