release: record the two backported #217 files as mapped dests

[yy-code-nv]

The forward release fails the drift gate with two collisions:

DEST BREAK collision: NEW source projects/cosmos3/cosmos3/utils/mixed_precision.py
  -> cosmos_framework/utils/generator/mixed_precision.py (already exists in CF, not previously mapped)
DEST BREAK collision: NEW source .../diffusion/samplers/step_callback_test.py
  -> .../diffusion/samplers/step_callback_test.py (already exists in CF, not previously mapped)

Both files were authored here in #217 (FP8 mixed-precision diffusion steps) and
backported into imaginaire4 as bfd655d67b. imaginaire4 now maps them, so the
release wants to take ownership of destinations this manifest never declared --
which the gate refuses on purpose, since that is also what an accidental
overwrite of community work looks like.

Declaring the pairs here is the acknowledgement: find_dest_collisions only
fires for a source absent from this manifest, so recording each pair states that
our copy of that destination now comes from the imaginaire4 source.

Verified against imaginaire4 main 173b1aeb83: the release goes from exit 1
with 2 collisions to exit 0 with zero breaks, with no other change.

Content impact of the adoption

• mixed_precision.py -- the released form is AST-identical to ours; the
  only differences are line wrapping from imaginaire4's 120-col formatter.
• step_callback_test.py -- additionally gains import pytest and
  pytestmark = [pytest.mark.L0, pytest.mark.CPU], which imaginaire4's conftest
  requires (it skips unmarked tests) and we do not use. Inert here: no markers
  are declared in pyproject.toml and --strict-markers is off. If you would
  rather not carry them, the alternative is a COSMOS-RELEASE-IGNORELINE on
  those lines in imaginaire4 instead of this adoption.

Provenance fields (_source_commit, _dest_commit, _generated_at) are
deliberately untouched -- no release has shipped yet.