fix max-column-alignment-gap to demote anchor when gap limit exceeded

[sirmspencer]

@weavejester I can't remember if you want an issue per PR, so I included the details here. I can make it an issue if you prefer.

LLM notice

I have reviewed all of what is being submitted. The PR description was partially generated with llm to show the current and expected.

Problem

Here, I am not sure if this is a bug or a differene in expected behavior so I went with a new flag.

When :max-column-alignment-gap is set, the alignment anchor is still the longest key in the form. Individual rows whose gap would exceed the limit fall back to a single space, but rows with medium-length keys are still extended all the way to the inflated anchor column. This produces inconsistent alignment: the shortest key falls back, medium keys over-extend, and the longest key (which caused the problem) sets the column for all of them.

Current behavior

Config: {:align-form-columns? true, :max-column-alignment-gap 5}

Input:

(let [a 1
      abcde 2
      abcdefg 3
      long-name 4]
  [a abcde abcdefg long-name])

Output — long-name stays as anchor; a falls back (gap 9 > 5); abcde and
abcdefg extend unnecessarily to the long-name column:

(let [a 1
      abcde     2
      abcdefg   3
      long-name 4]
  [a abcde abcdefg long-name])

The same behavior affects maps with :align-map-columns?.

Solution

This PR does not change the behavior of :max-column-alignment-gap. Instead it introduces a new flag, :max-column-anchor-gap, which applies the limit at anchor selection time rather than per row.

When :max-column-anchor-gap N is set, cljfmt selects the largest key for which every shorter key would need at most N spaces to align. Keys longer than the chosen anchor are not padded.

Expected behavior

Config: {:align-form-columns? true, :max-column-anchor-gap 5}

;; long-name (gap 8 from a) and abcdefg (gap 6 from a) exceed limit;
;; anchor demotes to abcde (gap for a = 4 ≤ 5)
(let [a     1
      abcde 2
      abcdefg 3
      long-name 4]
  [a abcde abcdefg long-name])

Composing both flags

The two flags are orthogonal and can be combined. :max-column-anchor-gap runs
first to select a conservative anchor; :max-column-alignment-gap then applies
per-row fallback to the result.

;; Config: {:align-map-columns? true
;;          :max-column-anchor-gap 5       ; long-key excluded; anchor = :abcde
;;          :max-column-alignment-gap 3}   ; further trims rows with gap > 3
;;
;; :x falls back (gap 4 > 3); :abc aligns (gap 3 = limit); :abcde is anchor
{:x "a"
 :abc   "b"
 :abcde "c"
 :long-key "d"}

Changes

• New option :max-column-anchor-gap added to default-options (nil by default)
• column-start-position applies cascade demotion when :max-column-anchor-gap is set
• pad-to-position gains a guard for keys wider than the demoted anchor, preventing
  their spacing from collapsing to zero
• README documents the new option and its relationship to :max-column-alignment-gap
• New test-max-column-anchor-gap deftest with form, map, and compose cases