fix(meade): restore hemisphere conversion for DEC coordinates

[ClutchplateDude]

Summary

The Meade parser refactor (#291) dropped the hemisphere conversion between the mount's internal DEC axis coordinate and celestial declination at the Meade protocol boundary. This restores it for all DEC paths crossing that boundary.

Background: the firmware stores DEC as a mount-axis coordinate — 0 at the pole above the mount, ±180 at the opposite pole — not as celestial declination. Every conversion to/from the wire format must apply:

• Northern hemisphere: celestial = 90° − |axis|
• Southern hemisphere: celestial = |axis| − 90°

The old code did this inside Declination::formatString() (output) and Declination::ParseFromMeade() (input). The refactor replaced those call sites with raw component extraction, dropping the transform. ParseFromMeade was left with zero call sites.

Symptoms

Command	Before this fix
:GD# / :Gd# (get DEC)	Reported the raw axis coordinate. Northern mount at celestial +80° answered +10*00'00#; southern mounts flipped the sign; values outside ±90° were possible
:Sd# (set target DEC)	Stored celestial DEC as an axis coordinate
:CM sync (DEC+RA)	Same as :Sd#

Slews commanded from Meade clients landed at the complement of the intended DEC. LCD menu, OLED, and :GX# status were unaffected (they use Mount::DECString()), which made this easy to miss.

Changes

• src/core/types/Declination.{hpp,cpp} — the transform now lives in pure, unit-testable statics: axisToCelestialSeconds(), celestialToAxisSeconds(), fromTotalSeconds(). Formulas are byte-for-byte the historical ones
• src/core/types/DayTime.{hpp,cpp} — splitSeconds(): signed seconds → (signed deg, unsigned min/sec)
• src/Declination.{hpp,cpp} — app overlay gains getCelestialDegrees() / fromCelestialDegrees(); formatString() now delegates to the shared core conversion instead of its own inline copy
• src/MeadeCommandProcessor.cpp — decFrom() (:GD/:Gd) applies the hemisphere correction; new decFromWire() used by onSetTargetDec() (:Sd) and onSyncCoordinates() (:CM)

Verification

• All 269 native unit tests pass, including 13 new ones pinning the full hemisphere relationship (poles, both equator crossings, sign-flip region, home-branch round-trips, clamping) — unit_tests/test_core/types/test_declination.cpp
• pio run -e ramps builds clean with -Werror
• clang-format 18 (CI version): no changes
• pio run -e esp32 fails in this workspace, but identically on the clean tree (missing RA_RX_PIN/DEC_RX_PIN in local config) — pre-existing, unrelated

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