* Re-org internal descriptor `CanonicalDesc` as `PivotArc`
why:
Despite its name, `CanonicalDesc` contained a cursor arc (or leg) from
the base tree with a designated block (or Header) on its arc members
(aka blocks.) The type is used more generally than only for s block on
the canonical cursor.
Also, the `PivotArc` provides some more fields for caching intermediate
data. This simplifies managing extra arguments for some functions.
* Remove cruft
details:
No need to find cursor arc if it is given as function argument.
* Rename prototype variables `head: PivotArc` to `pvarc`
why:
Better reading
* Function and code massage, adjust names
details:
Avoid the syllable `canonical` in function names that do not strictly
apply to the canonical chain. So renaming
* findCanonicalHead() => findCursorArc()
* canonicalChain() => findHeader()
* trimCanonicalChain() => trimCursorArc()
* Combine `updateBase()` function-args into single `PivotArgs` object
why:
Will generalise action for more complex scenarios in future.
* update `calculateNewBase()` return code type => `PivotArc`
why:
So it can directly be used as argument into `updateBase()`
* Update `calculateNewBase()` for target on parent arc
* Update unit tests
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