fix: don't double-process %% before a format specifier

[spokodev]

Problem

A literal %% immediately followed by a real format specifier is mis-rendered. The README states debug uses printf-style formatting and delegates %s/%d/%j to Node's util.format, so util.format is the source of truth:

const debug = require('debug')('app'); // DEBUG=*, colors off

debug('%%%s', 'X');        // logged: "app %s X"        util.format('%%%s', 'X')        === '%X'
debug('100%%%d done', 50); // logged: "app 100%d done 50"  util.format('100%%%d done', 50) === '100%50 done'

The output should match util.format. It only breaks for an even number of leading % before a specifier; odd counts (e.g. %%s) were already correct.

Root cause

src/common.js runs debug's own format pass before the result reaches the downstream logger:

args[0] = args[0].replace(/%([a-zA-Z%])/g, (match, format) => {
    if (match === '%%') {
        return '%';   // collapse here
    }
    ...
});

That collapsed string is then passed to util.formatWithOptions (Node) / console.* (browser), which collapse %% again. The escape is processed twice. For '%%%s', the first pass emits '%' + '%s' = '%%s', and util.format('%%s', 'X') is '%s X': the specifier boundary has shifted.

The reason debug collapses at all is that these loggers only collapse %% when at least one argument is present:

util.format('100%%')        // '100%%'   (0 args, not collapsed)
util.format('100%%', 'X')   // '100% X'  (1 arg, collapsed)

So with zero remaining arguments debug has to collapse %% itself, otherwise it would leak as %%.

Fix

Decide per-call whether the downstream logger will still have arguments:

• arguments remain → leave %% verbatim and let the logger perform the single, correct collapse.
• no arguments remain → collapse %% → % ourselves, as before.

The remaining-argument count is the original argument count minus those consumed by debug's own custom formatters (%o/%O/registered), computed before the pass so a %% appearing before a custom formatter is decided correctly.

util.format('%%%s', 'X')        // '%X'        ✓
util.format('100%%%d done', 50) // '100%50 done' ✓

This keeps %s/%d/%j, custom formatters, separated %% %s, zero-arg 100%% done, and %% inside custom-formatter output all correct.

Relationship to #1028 / #770

Both rewrite the custom-formatter output branch so a % produced by %o/%O is not re-interpreted (issue #766). They leave the %% escape branch untouched. Applying #1028 on top of current HEAD, debug('%%%s', 'X') still logs %s X: a separate defect in the %% branch, which is what this PR addresses.

Tests

Added a node test group in test.node.js asserting util.format parity for %% before a specifier, plus the preserved behaviors (zero-arg collapse, custom formatters, formatter-output verbatim). The parity tests fail on master and pass with the fix. test:node (20) and test:browser (14) are green.