feat(number_format): parse a format code and format a number with it

[andiwand]

🤖 Generated with Claude Code

Step 1 of the number-format plan (decision 14).

Change

New src/odr/internal/number_format/: Format parses a format code, and Format::format turns a number or a text into the text a cell shows. No reader uses it yet.

• Grammar: MS-XLS 2.4.126, the ABNF in offline/documentation.
  • Up to four sections, split on ; outside quotes and brackets.
  • [>=1000] conditions; [$€-407] shows its symbol; a colour and [DBNum1] change nothing.
  • 0, #, ?, the point, %, E+/E-, / fractions, @ and General.
  • Literals: "…", \x, _x as a space, and *x as nothing, because a fill needs the cell width.
• Commas: a comma between integer placeholders groups the integer. A comma after the last placeholder divides by a thousand. Any other comma is a literal.
• Sections: one section for all numbers. Two: non-negative, then negative without its minus. Three: positive, negative, zero. The fourth takes text. With a condition, the first section that matches wins. A section for the negative values drops the sign. A value that rounds to all zeros shows no minus.
• Rounding: half away from zero, on the 15-digit decimal spelling. So 2.675 with 0.00 shows 2.68, as a spreadsheet does, not 2.67.
• Integer placeholders: they fill right to left, and the leftmost takes the extra digits. So 000-00-0000 works.
• Scientific notation: ##0.0E+0 steps the exponent by three.
• Fractions: the best denominator within the placeholder digits, or a fixed one such as ?/8.
• General: 15 significant digits, and scientific notation outside [1e-10, 1e15). So 0.1+0.2 shows 0.3.
• Dates and times: a token is refused with std::invalid_argument until the next step.

Review after the rebase

• An infinity or a NaN threw out of std::stoi, because fmt spells it without an exponent; it shows as fmt spells it now.
• A mantissa of an engineering format that rounds into the next power stayed on the old one: 999.99 with ##0.0E+0 showed 1000.0E+0, now 1.0E+3.
• Format::sections had no caller and goes.

Both new test cases fail on the old code.

Proof

• NumberFormat.*: 11 tests, 0 failed, with the two cases of the review. They cover each rule above, with the cases the spreadsheets themselves show.
• Homebrew clang and g++-15 with -Wall -Wextra -Werror are clean on both new files. The run caught a missing designated initializer, which the type now defaults, and a trigraph in a test string.
• clang-tidy is clean on number_format.cpp.