Fix float parsing of ISO section codes like 5.10

[VivekGhantiwala]

What

Replace get_all_records() with get_all_values() in read_spreadsheet() to prevent gspread's numericise() from
converting ISO section codes like "5.10" to float 5.1.

Why

get_all_records() internally calls numericise() which strips trailing zeros from decimal-looking strings. The
existing numericise_ignore workaround misses column 0 and depends on gspread internals that have known bugs (see gspread#1007).
get_all_values() always returns raw cell strings, so "5.10" stays "5.10". We manually construct the row dicts using
the header row.

Testing

Existing test test_read_spreadsheet_iso_numbers validates that section IDs "1.10" and "10.10" are preserved as
strings.

Closes #574
Closes #546