Preserve all formatting on YAML --preserve set (no more flow-style collapse)

[machado144]

Problem

pipekit <fmt> set --preserve promises a surgical edit — change only the targeted value, touch nothing else. That held for editing an existing single-line scalar, but inserting a new key (e.g. bumping a chart's image.tag when the key isn't there yet) fell through to the node re-encode fallback, which reformats the entire document:

• Multi-line flow-style collections collapsed to one long line:

  args: [
    '--config',
    '/etc/app.yaml'
  ]

  became args: ['--config', '/etc/app.yaml'].
• Blank lines got dropped.

The data was always correct (it re-parses to the same structure), but the diff was far noisier than the one intended change — exactly the thing --preserve exists to avoid.

Fix

Add a surgical insert path between the scalar-splice fast path and the re-encode fallback. It:

• Anchors at the deepest existing block-style mapping along the path.
• Splices in only the missing tail — a single leaf line, or a nested block when intermediate maps are missing too.
• Mirrors the sibling key indentation; leaves trailing blank lines below the new entry.
• Validates the result by re-parsing and confirming the value resolves at the requested path before accepting it.
• Defers to the existing safe fallback for anything it can't do surgically (flow-style parents, non-scalar values, indexing into a scalar).

Net effect: everything outside the inserted line(s) stays byte-for-byte identical.

Before / after

Inserting .image.tag=v1.4.0 into a values file that has a multi-line args: [ ... ] and podAnnotations: { ... } elsewhere now produces a single-line diff (+ tag: v1.4.0); previously every multi-line flow block collapsed.

Tests

New byte-for-byte coverage:

• multi-line flow style ([...] and {...}) preserved on insert
• blank lines / comment alignment around the insertion point
• top-level key insert, nested-map creation, deep nested creation
• ambiguous-string quoting on insert (enabled: "yes")
• no-trailing-newline insert
• insert-under-a-scalar surfaces an error (defers to fallback)

All 275 tests pass; go vet clean.

🤖 Generated with Claude Code