cargo-anvil: finish moving table adoption off the line-oriented scanner

[Vaiz]

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Split out of #140. Re-scoped 2026-09-03: the defect originally reported here was fixed in #140 (084b42b) and is no longer part of this issue. What remains is a refactor with no known defect behind it.

What was fixed in #140

Adoption compared normalized source lines, so a hand-written workspace=true never matched a rendered workspace = true; adoption declined and the duplicate [lints] was appended. The comparison now goes through toml_edit and compares canonical path/value pairs, so spacing, entry order and quoting no longer affect the outcome. spacing_around_the_assignment_does_not_defeat_adoption pins it, and was observed failing against the pre-fix commit.

That fix also required two guards, both tested: table paths are compared as segments so ["a.b"] and [a.b] cannot collide, and the array-of-tables exclusion is applied in the rewrite as well as in candidate selection, since [bin] and [[bin]] now share a canonical path.

What remains

The comparison stage is parser-backed. The location and rewrite stages are still line-oriented, and that is what this issue now covers:

• Table headers are still found by scanning for a line that starts with [ and ends with ]. That is why the coarse multi-line-string guard has to exist: a bracketed line inside a """ value would otherwise be read as a header. The guard declines adoption for the entire host whenever """ or ''' appears anywhere in either input — correct, but it disables the feature rather than handling the case.
• The rewrite streams with independent in_managed and dropping booleans. Two of the defects found during review of fix(cargo-anvil): adopt an unmanaged TOML table instead of duplicating it #140 came from that pair ([[bin]] elements deleted; stale dropping surviving a managed-region boundary and removing unrelated user text, fixed in 81b85d6). The states are mutually exclusive and would be better as one enum — or absent entirely.

Proposal

Complete the move: obtain byte ranges for the approved ordinary tables from the parsed document rather than by scanning, build the non-overlapping deletion ranges up front, and produce the output by copying the gaps between them. This removes the streaming flags, and makes the multi-line-string guard unnecessary — restoring adoption for hosts that merely contain one, which today it refuses wholesale.

If parsing and source-range mapping disagree, return a conflict rather than deleting text.

Acceptance

Keep the regression coverage from #140. Add a host that contains a multi-line string and an adoptable table, which today declines and should then succeed.

Credit: raised in review of #140 by the sandersaares Copilot reviewer.