Invalidate declaration state when linked packages change - #64841

Fixes #64838

Incremental builds kept old declaration diagnostics and skipped d.ts emit for unchanged files even when the package links behind declaration module specifiers had changed, so tsc -b could disagree with tsc -b --force.

The snapshot now stores a signature of the program's symlinked package directories plus each linked package's entry fields (type, main, types, typings, typesVersions, exports), written to tsbuildinfo with paths relative to it. When the signature changes, every file gets its declaration emit queued again, the same way an emit affecting option change does. Dependency names show up through the links they resolve, and edits like a version bump leave the signature alone, so they don't trigger another emit. Unchanged d.ts text still skips the write, so downstream projects don't churn.

JSONValue gets a MarshalJSONTo so exports and typesVersions can be serialized for the signature.

Tests cover adding and removing a linked dependency, declaring a dependency on an existing link, and a linked package's types entry moving under tsc -b, plus tsc --incremental and watch. Three existing baselines just pick up the new tsbuildinfo field.

Not covered here: with --build, a link change with no manifest edit still doesn't mark the project out of date. That's the existing up to date check, which misses link only changes for module resolution too, so it seemed better as a separate change.

I used Claude Code while working on this and reviewed the changes.