Chart: orientation indicator + flag NCP-up fallback when GPS isn't ready (#419)

[brickbots]

Closes #419.

Summary

• The chart used to silently fall back to NCP up when the user picked Horizontal or EQ (Auto) but no GPS lock was available, then flip to the real orientation once GPS arrived. Since "horiz" is the shipped default (default_config.json:19), this is the normal first-run experience and was confusing.
• Adds a small bottom-right label on the Chart screen showing what's at the top of the chart — Zenith up, NCP up, or SCP up — so the user can always see the current orientation.
• When the resolved orientation is a fallback (Horizontal or EQ-Auto chosen but no GPS lock yet), the label is prefixed with ! and rendered at higher brightness, flagging that the chart will reorient once GPS comes online.

Changes

• python/PiFinder/ui/chart.py
  • New ChartOrientation dataclass (rot_deg, up_label, is_fallback).
  • get_chart_rotation_angle now returns it. Resolution is unchanged for eq_north_up / eq_south_up and for the GPS-locked branches of horiz / eq_auto. The lockless horiz and eq_auto branches now use location.lock (matching the canonical altaz_ready() test in state.py) and set is_fallback=True so the UI can flag the NCP-up default as transient.
  • UIChart.update draws the label after the chart paste.
• python/PiFinder/ui/align.py — caller updated to unpack rot_deg from the new return type. No behavior change for the align screen itself.
• python/tests/test_chart.py — new file, 11 unit tests covering each coord-sys × GPS-present/absent case plus the None RA/Dec path.
• .claude/skills/pifinder-remote/SKILL.md — documents the two one-time worktree-setup steps the headless launcher needs but git worktrees don't inherit (copy the Hipparcos catalog and the tetra3 submodule contents in from the main checkout), plus the venv-activation requirement so the launcher's find_python picks up the main checkout's interpreter via sys.executable.

Test plan

• pytest -m unit tests/test_chart.py — 11 new tests pass.
• pytest -m unit overall — 159 tests pass (148 pre-existing + 11 new).
• ruff check clean on chart.py, align.py, test_chart.py.
• Live UI: launched headless from a fresh worktree, navigated to Chart, screenshot shows Zenith up rendered at bottom-right alongside the optional RA/Dec line at bottom-left. (Headless --camera debug provides a fake GPS lock, so the rendered case is the non-fallback path; the !NCP up fallback rendering goes through the same code with different text + brightness — both unit-tested.)
• On-device confirmation: power up indoors (or otherwise pre-lock) with default config (chart_coord_sys = "horiz"), open Chart, confirm !NCP up shows immediately, then Zenith up after GPS lock.

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