PHASE 13.27.DF Commit 1 — Facet refactor (profile-only, channel-framework dispatch)

Replaces the inline facet=True path in DFDraw.profile() with a unified
_dispatch_faceted_render() method routing through the channel framework
(Phase 13.26). facet_by= is the new public API; facet=True is preserved as
backward-compat alias normalized to facet_by='group_by' (AD-67).

- DFDraw._dispatch_faceted_render() coordinates faceted rendering:
  validates facet_by, builds subplot grid, per-subplot recursion into
  draw_profile with filtered DataFrame + pre-created ax.
- DFDraw.profile() gains facet_by: Optional[str] = None. Commit 1 valid
  values: {'group_by', 'vector', 'quantiles'}. 'selection_delta' /
  'weights_delta' reserved for Commit 2 (NotImplementedError).
- Vector-y entry intercepts facet_by='vector' BEFORE _draw_vector
  (vector overlay and vector facet mut  (vector overlay and vector facet mut  (vector overlay and vector roup_by ≡ f  (vector overlay and vector facet mut  (vector overlaetLegacyEquivalence).
- Mutual exclusion: facet_by + same=True raises ValueError (architect rule
  §5.4 v  §5.4 v  §5.4 v  §5.4 v  §5.4 v  §5.4 v  §5.4 v  §5.4 v w='warn' truncates
  + warns, ='error' raises.
- 6 new style keys (AD-61..AD-66): channels.cycles.facet_max,
  channels.legend.facet_position, channels.label.{selection,weights}_truncate,
  channels.default.{selection,weights}_delta.
- 'facet_by' added to _PROFILE_FORWARDED_NAMES; R6 validator passes at
  module import (Phase 13.16 FIX1 + Phase 13.28 FIX1 lessons applied
  prospectively).
- Stale root-level dfd- Stale root-level dfd- Stale root-level dfd- Stale rdocs/dfdraw_Technic- Stale root-level dfd- Stale root-level dfd- Stale root-lever QRC v1.30 Rule 14:
- TestFacetLegacyEquivalence (3)
- TestFacetByCh- TestFacetByCh- TestFacetByCh- TestFacetByChSameTrueExclusion (1)

Tests: 662 passed, 1 skipped, 1 FAILED, 0 errors.

Deferred to Phase 13.30 commissioning:
  tests/test_channel_assignment.py::TestChannelStyleOverride::test_default_style_has_all_10_keys

  This test is a Phase 13.26 invariant that asserts an exact set of 10
  channels.* style keys. Phase 13.27 Commit 1 legitimately adds 6 new
  channels.* keys (AD-61..AD-66 above), so the assertion is correctly
  identifying the new state — but the expected set is stale.

  The fix (rename to test_default_style_has_all_channels_keys + bump to
  16 keys grouped by phase with comments) bundles with Phase 13.30
  closure to avoid an extra commit cycle. After Phase 13.30 lands the
  suite returns to a green baseline of 675 / 1 / 0.

Author: miranov25