feat(csv): tell a number from a code

[andiwand]

🤖 Generated with Claude Code

Last of five stacked PRs. Based on #668. Plan: src/odr/internal/csv/PLAN.md.

ValueType is only {unknown, string, float_number} (document_element.hpp:135) and drives exactly one CSS class — text-align:right (html/frontend.cpp:32). So this stays small on purpose.

Per column, not per cell

A reader compares down a column, so one number in a column of prose is not a quantity:

a,b
x,note
y,42     <- string, not a number

The first row is left out of the sample: a header names its column rather than holding a value, and one word would otherwise make every column prose. The header of a numeric column stays text for the same reason.

Empty cells do not break a numeric column.

What the grammar refuses is the point

input	verdict	why
42, -3.14, 1.5E-3	number
007, 0123456789012	string	a leading zero is what tells a code from a quantity
1,234 / 1.234,56	string	a thousands separator does not say which side of the Atlantic wrote it
2026-08-09, 03/04/2026	string	03/04/2026 is two different days; guessing gets it wrong confidently

Dates are left alone entirely rather than half-supported.

Quoting is not consulted. RFC 4180 quoting is lexical — Excel quotes any field holding a separator, a quote or a newline, and plenty of writers quote everything — so it carries no type information. This was the one item in the original brief I pushed back on, and it stays pushed back.

The raw string is always what gets displayed; the value type only changes alignment.

Tests

is_number is a free function and tested directly, plus the column rules through the sheet. 577 unit tests and 234 reference-output tests green.

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