MetadataFile.FindLocation(anchor, needle) points the diagnostic at the wrong entry when the needle's key comes before the anchor's key in the same JSON object #66

What's wrong

FindLocation(string anchor, string needle) (SourceGeneratorToolkit/MetadataFile.cs:109-124) finds the anchor and then searches forward from it for the needle (IndexOfToken(needle, scope)). JSON objects have no key order, so the property being reported (for example base) can come before the entry's identifying key (for example name). In that case the needle in the anchor's own entry is never found, and the method returns either:

• the needle in a later entry, or
• if there is no later one, the unscoped first occurrence, which can be an earlier, different entry.

In both cases the result is a valid-looking location, so the generator's diagnostic squiggles the wrong line. The overload's remarks say it exists precisely so that a name "spelled correctly in dozens of places and only wrong in one of them" points at the right entry.

Reproduction

{"units":[
{"base":"Meter","name":"Foot"},
{"base":"Meter","name":"Yard"}
]}

• file.FindLocation("Foot", "Meter") returns the location on line 2, the Yard entry. The expected result is line 1. This was checked with a probe test in a built copy of the repo.
• Fallback variant: with entries Inch then Foot, both written base-first, FindLocation("Foot", "Meter") finds nothing after Foot and falls back to the first Meter, which is in the Inch entry.

The existing test ScopedFindLocationFallsBackWhenTheNeedleIsOnlyBeforeTheAnchor only asserts that the fallback equals the unscoped result. No test asserts that the anchor's own entry is chosen when the needle comes first in it.

Suggested fix

Scope the search to the JSON object that contains the anchor:

1. Walk back from the anchor to the { that opens its enclosing object, skipping string contents and balanced nested {…}. A plain LastIndexOf('{') breaks when there is a nested object before the anchor.
2. Search for the needle between that { and its matching }.
3. Fall back to the current forward-from-anchor search, then to the unscoped search, only when the needle isn't inside that object.

Acceptance criteria

• In the example above, FindLocation("Foot", "Meter") returns line 1.
• In the Inch/Foot variant, it returns the Foot entry rather than falling back to Inch.
• Existing tests still pass: anchor-first order, the substring cases from MetadataFile.FindLocation(anchor, needle) points at the wrong entry when the anchor or needle is part of a longer name ("Meter" lands on "SquareMeter") #33/Find a whole JSON string before a substring in FindLocation [patch] #63, and needle-absent fallbacks.