[ax] Add per-line rule attribution to JSON output

[TomasVotruba]

Why

The --output-format=json payload lists applied_rectors as a flat, de-duplicated array of class strings per file. It drops the line each rule fired at - even though Rector already tracks it internally (RectorWithLineChange carries {rector_class, line}). Machine consumers (agents, editors, CI annotations) had to re-parse the raw diff blob to guess which rule touched which line.

What

Emit an additive changes array on each file diff, pairing every rule application with its line:

{
    "file": "some/file.php",
    "diff": "--- Original\n+++ New\n@@ -38,5 +39,6 @@\n...",
    "applied_rectors": [
        "Rector\\Php80\\Rector\\Identical\\StrEndsWithRector",
        "Rector\\Php80\\Rector\\Identical\\StrStartsWithRector"
    ],
    "changes": [
        { "rector": "Rector\\Php80\\Rector\\Identical\\StrStartsWithRector", "line": 38 },
        { "rector": "Rector\\Php80\\Rector\\Identical\\StrEndsWithRector", "line": 42 }
    ]
}

• applied_rectors is kept unchanged - backward compatible, no consumer breaks.
• changes reuses FileDiff::getRectorChanges(), which already holds the per-line data; only a getLine() getter was missing on RectorWithLineChange.

line is the node's source line when the rule fired, not the post-change hunk line - approximate but enough to jump to or group by rule. Structuring the diff into machine hunks is a larger, separate change.

Tests

• New testReportShouldAttributeEachRectorToItsLine with a with_diffs.json fixture.
• composer check-cs clean, existing JSON test still green.