Release 2.0.0

[Fivell]

Merge this last. #55 and #56 both add entries under ## [Unreleased]; once they land I move those entries under the ## [2.0.0] heading here and rebase. Merging this first would date a release that does not yet contain them.

Why major

The selectors build a different string than 1.0.0 did for the same input, so a suite that passed on 1.0.0 can fail on this version without a line changing:

input	1.0.0	2.0.0
'E-mail'	col-email	col-e-mail
:full_name	col-fullname	col-full_name
"Customer's Name"	col-customers_name	col-customer_s_name
'Billing::Employee'	billing::employee (raises)	billing_employee

That is a breaking change by SemVer, and the gem's README commits to SemVer. 1.0.0 → 2.0.0.

The sharpest case is a consumer who worked around the old behaviour: a helper that pre-normalized labels before calling these selectors passed on 1.0.0 and breaks here, because normalizing twice strips the separator the first pass inserted. That is called out in the migration notes and in the new README section.

What is in this PR

• version.rb 1.0.0 → 2.0.0
• CHANGELOG.md: a dated ## [2.0.0] heading
• README.md: a new How labels become selectors section

The README section

The README had no reference for any of this — spec/support and test_helpers.rb were the only pointers, and neither explains how a label turns into a selector. Three things cost a downstream suite a day today, and all three were invisible from the docs:

1. Do not normalize the label yourself. The gem makes the same parameterize(separator: '_') call ActiveAdmin makes; doing it first corrupts the result.
2. row :salary, class: 'money' makes ActiveAdmin emit no row-* class at all (if options[:class] … elsif title.present?), so no label can address that row. Match on text.
3. have_attributes_table(model:) follows the model class, have_table(resource_name:) follows the registered resource name. register Billing::Employee, as: 'Business Employee' splits the two, and passing the class to the second one silently builds index_table_billing_employees for a table rendered as index_table_business_employees.

Plus the label→class table, and the two cases where there is no class to match (own class:, and a label parameterize cannot transliterate).

Verification

Every selector in the new table was produced by running the gem, not written from memory:

"Full Name"          -> col-full_name
:full_name           -> col-full_name
"VAT / TAX Number"   -> col-vat_tax_number
"E-mail"             -> col-e-mail
"Customer's Name"    -> col-customer_s_name
"# of DIDs"          -> col-of_dids

The two model-name examples were read off real rendered pages earlier today: the dummy renders <div class="attributes_table billing_employee"> and <table id="index_table_business_employees">.

Suite: 34 examples, 0 failures on this branch (42 once #55 and #56 are in).