[Mehulsri07]

Phase 2 of the design: the calculation behind everything StrataVision will show. No screens use it yet, so the app looks the same as after phase 1.

What it adds

• Drilling groups. Every soil type falls into clay, fine sand, water-bearing or hard. The driller's water-bearing tick upgrades fine sand. "Not recorded", gaps and unlinked soil names never count.
• The vote (predictColumn). For a point, in 5 ft steps: nearby borewells that reach that height vote, closer ones count more, poor records and address-located borewells count less. Borewells at one spot share a vote and take at most 3 of the 12 seats. Layers are compared by height above sea level, not depth. Each step says the share for each group, the likeliest, a confidence (likely, rough, none) and which borewells it came from.
• The accuracy check (checkModel). Hides each borewell in turn, with everything else at its spot, predicts it from the rest and counts how often the best guess was right, split by confidence and by distance. Also lists odd ones out: borewells that disagree with their surroundings far more than usual.
• The soil-type list now comes with the records, because hand-added soil types are only known to the database.

On a made-up city of 500 borewells

The true ground is known, layers thin out and vanish from place to place, and 30% of borewells share a spot with another.

	Share right
Guessing by height alone, ignoring the place	71%
Steps the model calls "likely"	95%
Steps the model calls "rough"	79%
Likely and rough together	92%

With 50 of the 500 logs typed in upside down, the check listed 40 of them as odd ones out, along with 7 honest ones.

More borewells gave a better score (100 vs 500).

These logs are clean and exactly placed, so real records will score lower. The constants get tuned against real records in phase 3.

	Time
One column	under 1 ms
200 columns (a section)	about 65 ms
10,000 columns (a city map)	about 2.5 s
Accuracy check on 500 borewells	about 0.4 s

Corrected after review

• Layer depths are always feet in StrataField (it converts logs written in metres when it reads them). The first version converted again for old records marked "m"; that is removed.
• One spot with many logs could fill all 12 seats; now it gets 3.
• The check used to let a borewell be vouched for by another at the same spot; now the whole spot is hidden.

Checks

60 TypeScript tests (new in this PR; CI now runs them), 8 Rust tests, the start-up test of the real app.