Guard unsaved Library Source edits during navigation

[poitee]

A Library Source's Import files and Naming editors use explicit Save buttons. If the user changes either draft and opens another Source or leaves Library, the draft currently disappears without a discard decision.

Register the sheet's dirty state with a small Library context and extend the existing single navigation blocker. Tab and file query changes within the same Source remain available. Route exits and Source switches ask whether to discard, then either keep the current editor or continue navigation. The sheet's close prompt and browser reload warning remain in place.

Isolated Chrome proof: with Source A's import selection changed, cancelling a Source B switch kept Source A and its 0/1 draft; accepting switched Sources, and the Source A API still held the saved selection. With a Naming override changed, cancelling navigation to Builds kept Library and the unchecked draft; accepting opened Builds. Focused guard tests cover both cancellations and acceptance. Web suite: 264 files and 1,396 tests passed with --maxWorkers=2. Monorepo typecheck, lint, and diff check passed.

This branch is stacked on #91. It also contains cherry-picked #87 data-router commits through bc0a799c, since the shared blocker depends on them. The new GRE-301 change is commit da10fe98; the #87 commits can be deduplicated when the stack is landed.

Tracks GRE-301.

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