Linked workbooks: references typed by file name or path go through a link, shown with the file

[marcoarnulfo]

Fixes #804.

Built on #464 (it needs its ExternalBook and the reading of the link parts), so until #464 is merged this branch shows its three commits too; the last three commits are this PR. It sits next to #824 (#801): that one adds Workbook::source_dir and rebases kept link targets on Save As, and this one adds ExternalBook::path. They touch passthrough.rs and its tests in different places, and I'll rebase whichever lands second.

The model and the file keep only [n], the position of the link. What changes is the way in and the way out. When a formula or a name is entered, a reference to another workbook typed by file name or path ([Source.xlsx]Sheet1!A1, 'C:\links\[Source.xlsx]Sheet1'!A1, Source.xlsx!Rate, 'C:\links\Source.xlsx'!Rate) is rewritten as [n]Sheet1!A1 / [n]!Rate. The link is matched by path, or by file name when no folder is given. Otherwise a new link is added with the typed path. A new link has no values yet, so its sheets are marked refreshError, and the reference is #REF! as in Excel when the file is missing. ExternalBook now keeps the link's path, read from the externalLinkPath relationship (file:///C:\links\Source.xlsx → C:\links\Source.xlsx, a relative target stays relative). The formula bar, Show Formulas, the Name Manager and document.inspect print [n] back as the file ('C:\links\[Source.xlsx]Sheet1'!A1), and that text goes back in unchanged when you press Enter. On save, the links read from the file are still passed through as they were. Each new link gets its own externalLink part and rels after them, so [1] stays [1]. <f> only ever holds [n].

In the formula language: the unquoted [Source.xlsx]Sheet1!A1 now lexes (it was refused with unexpected Sheet("Sheet1", None)), and a drive's C: inside a quoted path is no longer read as the colon of a 3-D range. That colon is what produced the 'C:\…\[links[Source.xlsx]Sheet1]…' text Excel showed. [Source.xlsx]Sheet1 prints unquoted when the sheet name allows it.

Choices I'd flag (happy to switch):

• A link read from the file with a relative target (Target="Source.xlsx") is shown as [Source.xlsx]Sheet1!A1. Excel shows the absolute path there, resolved against the workbook's folder. With XLSX: Save As into another folder keeps links on the same workbook #824's source_dir that's a small follow-up, so I left it out here.
• Book.xlsx!Name is read as another workbook only when the qualifier isn't a sheet of this workbook and has a folder or a file extension. So Data.v2!Rate stays local when there's a sheet Data.v2, and Plain!Rate isn't treated as a file.
• If you type a sheet or name that an existing (kept) link doesn't have, it's added to the model, so the reference is #REF!. It isn't added to the kept part, which is written byte for byte. A new link's part is generated from the model.
• Reading the linked file (Update Values / Edit Links) is still out, as the issue says.

Validation:

• linked_workbooks.rs (engine, new), one assertion per row of the issue's table: what is kept ([1]Sheet1!A1, SUM([1]Sheet1!A1:A3), '[1]P&L Data'!A1, [1]!Rate), the value (#REF! for a new link; 10, 20, 30, 60 and 0.05 through an existing link), the saved <f> and the externalLink part with its rels target, the formula bar text before and after reopening, and the Name Manager / document.inspect refersTo. It doesn't build on XLSX: formulas into linked workbooks calculate from the values saved with the link #464's head (no path in ExternalBook). With the entry rewrite removed, it fails as on main ([Source.xlsx]Sheet1!A1 kept instead of [1]Sheet1!A1).
• another_workbook_by_file_name_or_path_parses (formula): fails on XLSX: formulas into linked workbooks calculate from the values saved with the link #464's head ([Source.xlsx]Sheet1!A1: unexpected Sheet("Sheet1", None)), passes here.
• a_new_link_follows_the_kept_ones (xlsx): a kept link plus a typed one are written as [1] and [2], both read back with their paths. damaged_kept_content_is_dropped_without_failing_the_read still holds: when the file's links can't be kept, nothing is generated after them.
• Mutations, each one caught: no rewrite on entry; always a new link; [n] in the formula bar, the Name Manager or inspect; a defined name kept as typed; any qualifier read as a file; no new link part; the path not decoded; a new link without refreshError (0 instead of #REF!); a drive colon splitting the sheet; the file name quoted by the printer.
• cargo test -p gridcraft-formula --lib, cargo test -p gridcraft-xlsx --lib a_new_link_follows, cargo test -p gridcraft-engine --test linked_workbooks, cargo test -p gridcraft-xlsx --lib passthrough_tests, cargo test -p gridcraft-calc --lib linked_workbooks, cargo clippy -p gridcraft-formula -p gridcraft-model -p gridcraft-calc -p gridcraft-xlsx -p gridcraft-engine -p gridcraft-ui-egui --all-targets -- -D warnings, cargo fmt --all -- --check
• Not checked in Excel: I wrote the new link part the way Excel writes one for a missing file (sheetNames, definedNames with no refersTo, refreshError sheet data, file:/// path for absolute paths). Opening it in Excel 365 is the check I can't do here.
• (macOS, Apple silicon)

[marcoarnulfo]

Rebased onto current main. One commit is new rather than a conflict fix: after #567 the printer quotes sheet-qualified names, which turned a linked workbook's name [1]!Rate into '[1]'!Rate (and quoted a typed path twice). 48d4342 keeps #567's quoting for sheets and leaves the book-only [1]!Rate form bare, with a parse → print → parse check in the formula tests; quoted_sheet_qualified_name_keeps_quotes still passes.