Store the wide decimal on the sixteen byte plane

[tamnd]

Closes the note left open on the decimal row of S2 (#700).

A decimal was already one declared type: the column holds unscaled units and the scale in the catalog is what makes them a number. This makes it one declared type stored two ways. Up to eighteen digits the units ride a lane word, as they have since #765. Above that they are sixteen little endian bytes at a fixed stride, which is the plane INT128 opened in #777 and the layout BINARY(16) has always used. Nothing new was needed on the storage side: fixed_octets is the single hook that puts a column on that layout, write_columns is generic over it, and answering Some(16) gets padding, the segment write and the reader for free.

The precision in the declared type is what decides which of the two a column is, and writer and reader both ask fixed_octets rather than deciding for themselves, so they cannot disagree about a column they are looking at together.

Where the frontier is now

At the value carrier, which is where the wide integers landed too. Value::Decimal carries an i128 of unscaled units, thirty eight digits is the widest an i128 holds, so thirty eight digits is the widest a value of one can be. DECIMAL(39,2) has no declared form and is refused at the declaration with 42000, the same place and the same condition the wide decimal's own refusal used to land. Below that line every precision a statement can spell is a column a statement can fill.

One consequence worth naming: a bare DECIMAL is DECIMAL(38,0) by the parse cap, so a bare DECIMAL is now a declarable and storable property type. It was not before.

The example that had to move

DECIMAL(38,2) was the canonical spelled and not storable example in six places, and it stores now. All six moved to INT256, which is durable in a way the decimal was not: the decimal was one carrier away from storing and did store, whereas an INT256 column would take a row in and be unable to give it back, and that does not change until the engine has a two hundred and fifty six bit carrier. The six are session.rs, refusals.rs, refusal_shape.rs with its regenerated snapshot, catalog_statements.rs, graph_type.rs and the corpus case a-property-type-no-column-can-hold.

What is checked

check_declared reads sixteen octets back into a Decimal at the declared scale and compares its digit count to the declared precision, so a wide column is held to its declaration the same way the narrow one is, on the same code path the narrow one uses. frontier() in props.rs now pins DECIMAL(38,2) storable and DECIMAL(39,2) not, and the extended type test walks precisions 1, 9, 18, 19, 38, 39 and 76 and asserts which plane each is on. The new end to end test declares DECIMAL(32,4), inserts a twenty eight digit value, a rescale up from DECIMAL(5,1) and a bare integer, reads all three back in order, and pins the two 22003 refusals: thirty three digits, and a fifth decimal place.

The directory version goes to 14.

Local gates green: fmt, clippy with -D warnings, xtask terms, xtask api-map, and 89 test result: ok across the workspace with no failures.