Sidereal (LST) and apparent-solar time-row modes

[peterlewis]

Stacked on mitxela#6 (astro-pack) — review that first; this PR's own diff is just the sidereal/solar modes.

What this adds

Two opt-in time-row modes that turn the big HH:MM:SS digits into a live ticking clock in an alternate timebase:

• MODE_LST — Local Sidereal Time (the astronomer's clock; a GPS-disciplined sidereal clock is the appeal).
• MODE_SUNDIAL — local apparent solar time; reads exactly 12:00:00 at your meridian transit.

How it stays honest and leaves civil timekeeping untouched

It clones the existing MODE_COUNTDOWN takeover (new COUNT_ALT, four SysTick_Alt_Px handler clones). The heavy GMST/EoT double maths runs once per second in the main loop (thread context), staging pre-rendered 7-seg digits; the SysTick handlers latch them at the true PPS boundary via the untouched PPS() path. So:

• civil currentTime, PPS discipline, calibrateRTC/write_rtc, and the date row are byte-identical to stock;
• the setPrecision P3→P0 sub-second digit-hiding applies to sidereal unchanged (it degrades in lock-step with civil holdover because the reseed epoch is the disciplined currentTime);
• no double-precision maths ever runs in an ISR.

The display is quantised to civil second boundaries and reseeded each second. Sidereal's 1.00273791× rate therefore makes the seconds display double-step about once every 6 min — deliberately visible, the authentic signature of a GPS-disciplined sidereal clock rather than a rate-warped one.

Not mistaking it for civil time

A dedicated colon animation (alt_colon_mode, default alt_sawtooth) marks the alternate modes and is automatically kept distinct from the civil colon unless explicitly matched — sundial especially can sit within minutes of civil. With no position (no fix, no fake_longitude) the row shows dashes; it never displays GMST-as-LST.

Maths

astro.c regains local_sidereal_time() / local_solar_time(), with the GMST series factored into one shared gmst_hours() helper (plus the +T² term so truncation stays sub-ms for decades). The native suite is 53/53, including the IAU GMST(J2000.0) anchor and Meeus Astronomical Algorithms example 12.b. UT1−UTC (DUT1, ±0.9 s) is the documented accuracy floor.

Default-off: without MODE_LST/MODE_SUNDIAL in config, no alt handler is ever installed and behaviour is identical to stock.

Status

Builds clean (arm-none-eabi-gcc -O3, zero warnings in changed code). Passed an adversarial review pass (ISR-safety, quantisation, mode-machine). Hardware validation pending — flagging as draft until watched on the device.

[peterlewis]

Superseded by mitxela#8 (draft, targets upstream). Branch kept for the clean stacked diff against astro-pack.