Add semantic_model_diff_editor: interactive diff & cherry-pick UI for semantic models

[Copilot]

Adds an interactive UI (in the style of perspective_editor) for diffing two Fabric / Power BI semantic models and selectively promoting changes between them — a lightweight CI/CD workflow for semantic modelers.

from sempy_labs.semantic_model import semantic_model_diff_editor

semantic_model_diff_editor(
    source_workspace="Dev",   source_dataset="Sales",
    target_workspace="Prod",  target_dataset="Sales",
    format="TMDL",   # or "BIM" / "TMSL"
)

UI (src/sempy_labs/semantic_model/_diff_editor.py)

• Independent workspace + dataset pickers for source and target, with a swap button.
• Format segmented control: TMDL (per-file .tmdl) vs BIM (single model.bim).
• Color-coded file list (added / removed / modified / unchanged) with per-file +/- counts and summary stat chips.
• Click-to-expand inline unified diff per file; per-hunk checkboxes plus a tri-state aggregate per file.
• Apply selected → Target / Apply selected → Source (confirmation prompt) — destination is rewritten via the Fabric updateDefinition API; non-text parts (e.g. definition.pbidataset) are echoed back unchanged. Both sides auto-refetch and re-diff after apply.
• Bells & whistles: filter, expand/collapse all, select all/none, reload, download .patch, dark/light theme toggle.

Diff & apply pipeline

• Diff computed server-side per file via difflib.SequenceMatcher.get_opcodes(), producing line-level hunks tagged equal / replace / insert / delete.
• Apply semantics: for each modified file, kept hunks come from the destination and selected hunks are replaced with the other side's lines. Added/removed files are propagated/dropped according to direction.

Tests (tests/test_semantic_model_diff_editor.py, 73 tests)

• Functionality — pure helpers (_split_lines, _join_lines, _compute_file_hunks, _compute_diff, _apply_hunks_to_lines) hoisted to module level and unit-tested for empty/identical/insert/delete/replace cases, file-status classification, sort order, and select-none/all/partial cherry-pick in both directions.
• Design — public surface (signature, defaults, numpydoc docstring incl. Service-Principal note, package export, format validation, BIM alias); embedded CSS (scoped .slls-de-* prefix, light + dark themes, required selectors, semantic color tokens); embedded JS (anywidget ESM shape, every traitlet referenced from JS, every pending_action emitted, expected UI labels, escapeHtml present).
• tests/conftest.py — puts src/ on sys.path and stubs sempy._utils._log so the suite runs without semantic-link-sempy installed.