fix(export): write the IR TIFF sidecar without the source filename in its description

[MohammedAlkindi]

Closes #879

Implements the fix as scoped by @thetalkingdrum in the issue thread:

• _write_ir_tiff now writes a fixed ASCII description (NegPy Linear Output -- infrared channel.), matching _write_tiff's fixed-string style — no filename, so tag 270 can never see non-ASCII bytes.
• The imwrite is wrapped in try/except following the _write_tiff extratags precedent: a sidecar failure no longer kills the export.
• On failure the partial sidecar is unlinked, so no header-only _ir stub is left looking like a real file.
• The source_name parameter is dropped; the only caller passed os.path.basename(file_path) purely for the description.

Test: test_ir_sidecar_survives_non_ascii_source_filename copies the 4-channel LinearRaw DNG fixture to négatif_şcan.dng, exports as TIFF, and asserts the sidecar exists, is larger than a bare header, decodes as uint16, and carries no filename in its description. It fails on current main with tifffile's ValueError: TIFF strings must be 7-bit ASCII and passes with this change.

Testing: full suite on Windows 11 / Python 3.13: 4259 passed, 10 skipped, 11 deselected, 0 failed (tests/test_linear_output.py alone: 169 passed). ruff check (pinned 0.14.10) clean on both changed files.

Deliberately unchanged, per the issue discussion: the _build_xmp gate stays camera_wb-only, so the filename is not relocated into XMP.