Southern targets guide: objects invisible from the UK

Adds TARGETS.md, a target guide for the southern iTelescope sites (Siding
Spring Q62, Deep Sky Chile X07) built around UK visibility: from lat 51.5°N
anything below dec -38° never rises and dec -25° to -38° culminates under
~15° altitude. Targets are tiered accordingly (never visible vs hopeless),
grouped by region (Magellanic Clouds, Carina/Crux, Centaurus, Ara/Scorpius
tail, far-southern galaxies, SNR/planetaries), and each is matched to the
best scope from TELESCOPES.md with the field-of-view reasoning in the notes.

Includes a seasonal booking calendar (evening sky by quarter from the
southern sites, with the Q62 30-35° minimum-elevation caveat), a suggested
first campaign for July-August (free T33 globulars, T71 narrowband on Ara/
Scorpius, T73 on NGC 6744, then T8 on the Magellanic Clouds in Oct-Nov),
and cost-shape notes (fast scopes for nebulae, dark-of-moon for LRGB).

README links the new guide; state/TODO.md records the addition.

README.md

@@ -6,6 +6,8 @@ book for a given target.
 
 - **[TELESCOPES.md](TELESCOPES.md)**: the full review, grouped by observatory, with a
   "choosing a telescope" guide at the end.
+- **[TARGETS.md](TARGETS.md)**: southern-sky targets invisible from the UK, matched
+  to scopes and to the time of year to book them.
 - **[data/itelescope-telescopes.csv](data/itelescope-telescopes.csv)**: snapshot of
   the network's maintained specs sheet (source of truth for the numbers).
 - **state/**: project state under the Default Workflow.